[WIP] patcher concepts: what is a patch (#2180)

* patcher concepts: what is a patch

* PR rewords, new images

* Update patches.md

* Update docs/2.0/docs/patcher/concepts/patches.md

* Update docs/2.0/docs/patcher/concepts/patches.md

* remove example, fix screenshot

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Co-authored-by: Zach Goldberg <[email protected]>

Author: ceschae

custom-dictionary.txt

@@ -44,4 +44,6 @@ mimecast
 slugified
 dlist
 DEPENDENCYID
-MTTR
+MTTR
+terrapatch
+Terrapatch

docs/2.0/docs/patcher/concepts/patches.md

@@ -1,3 +1,97 @@
 # What is a Patcher Patch?
 
-This page is under construction
+A Patch is a set of instructions executed by Patcher that do code transformations.
+This strategy is especially useful as a way to automate adoption of a breaking change with infrastructure as code, such as Terragrunt, OpenTofu, or Terraform.
+This instruction sheet is delivered by means of a `yaml` file in a specific format:
+
+```yaml title=".patcher/patches/v1.0.0/my-patch/patch.yaml"
+name: "<name-of-patch>"
+description: "<description for patch>"
+author: <your-name-here> <<your-email-address-here>>
+
+# Optional dependencies. Terrapatch is a typical one
+dependencies:
+  - name: terrapatch
+    version: "0.1.0"
+
+# Steps necessary to resolve breaking change
+steps:
+  - name: "<name-of-step>"
+    run: <command-to-run>
+  - name: "<name-of-second-step>"
+    run: <second-command-to-run>
+  # etc
+```
+
+[Check out an example of a patch in the CIS Service Catalog.](https://github.com/gruntwork-io/terraform-aws-service-catalog/blob/c3d5ede211fc3230a7d493ceea43622b337ee88a/.patcher/patches/v0.96.4/switch-to-cis-rds-module/patch.yaml)
+
+## For Module Consumers
+
+As an OpenTofu/Terraform module consumer, modules are imported to launch infrastructure by writing Terragrunt units (`terragrunt.hcl` files) or OpenTofu/Terraform code that calls a module.
+
+It is best practice to reference a specific version of an OpenTofu/Terraform module. 
+Over time, module authors release new versions of the module, and the code that consumes those modules slowly gets out of date. 
+In some cases, the latest update of the underlying modules requires a breaking change to the consuming code, meaning the version can't just be bumped. 
+The consuming code needs to be edited to make use of the new version. 
+In this instance, using a patch with Patcher comes in handy.
+
+Patches can be consumed with either a "push" strategy, when Patcher proactively opens a pull request with the latest update, or a "pull" strategy, when a repo is manually scanned to look at the current state of your infrastructure using the Patcher CLI tool.
+
+Regardless of methodology, the concept remains the same. 
+Patcher will suggest changes to your codebase in order to keep your infrastructure up to date, however you see fit. 
+
+Here is an example of what your PR will look like, after being generated by Patcher:
+
+![Patcher PR Example](/img/patcher/pr-example.png)
+
+### Update Push Strategy
+
+Let's take a look at the experience of the "push" strategy available through our [GitHub action](https://github.com/gruntwork-io/patcher-action). 
+Using the GitHub action, PRs will be opened against your codebase on a cadence specified by you, against environments, versions, etc. specified by you. 
+The intention with this GitHub action is to leave the repo owner in full control of your upgrade cadence. Check out our guide on [promotion workflows](/2.0/docs/patcher/guides/promotion-workflows), so that updates can proceed from `dev` to `stage` to `prod` (or any other environment sequence) to mitigate risks around upgrades. 
+
+### Update Pull Strategy
+
+Let's take a look at the experience of "pulling" available updates by using the Patcher CLI tool. 
+The first step is to run `patcher update` within the repo in which updates are desired. 
+When `patcher update` is run, the default mode is to click through the updates **interactively**. 
+In this mode, available updates are found, and the details of those updates are presented to you:
+
+![Patcher Update Interactive Mode](/img/patcher/interactive-update.png)
+
+You can choose to run in `--non-interactive` mode, which will modify the codebase and present results about what the program did at the end. 
+
+By default a PR will _not_ be opened with the changes.
+However, the changes should be visible within the version control system. At that point, you may make a PR or apply the changes using your IaC system.
+
+### Examples Running `patcher update`
+
+Here's the easiest way to run this command:
+
+```bash
+$ cd <repo-root-directory>
+# Show what patches are available for everything in the current directory and all it's children
+$ patcher update ./
+```
+
+If more fine-grain controls are desired, the following example (which includes advanced usage topics like [update strategies](/2.0/docs/patcher/concepts/update-strategies.md)) has those:
+
+```bash
+# run 'update' non-interactively, only up to the next safe version, and publish a PR with the changes
+$ patcher update --update-strategy next-safe --non-interactive --publish --pr-branch grunty/update-via-patcher --pr-title "[Patcher] Update All Dependencies to Next Safe"
+```
+
+More details on the available options included in `patcher update` can be found in the [reference section](/2.0/reference/patcher/index.md#update). 
+
+## For Module Authors
+
+Module authors periodically need to introduce breaking changes in their modules, causing a downstream, potentially painful, experience for module consumers. 
+With patches, module authors include a patch YAML file that automatically updates consuming code to incorporate the breaking changes associated with the updated module code.
+Doing so allows module consumers to use patches to enable their modules consumers to automatically update consuming code to adopt breaking changes.
+
+In a Patcher ecosystem, the resolution to such a change is written once, in a patch, and distributed to all consumers. 
+Although your release will succeed with or without a patch, downstream consumers of your breaking change will praise you thoroughly for your advance work. 
+
+In theory, you may write whatever command execution steps you want to perform patch steps. 
+For example, there are many cases where validating tool versions are required, or using `sed` to find and replace certain values. 
+However, we _strongly_ recommend using [`terrapatch`](https://github.com/gruntwork-io/terrapatch), a Gruntwork tool that surgically updates Terraform/OpenTofu HCL files.