DEV-666 Add Running Plan/Apply Guide (#2208)

* Add Running Plan/Apply Guide

* Add Destroying Infrastructure section

Author: Resonance1584

docs/2.0/docs/pipelines/guides/running-plan-apply.md

@@ -1,3 +1,46 @@
 # Running Plan/Apply with Pipelines
 
-This page is under construction.
+When changes are made to your committed IaC, Pipelines detects these infrastructure changes and runs Terragrunt Plan/Apply on your units. Changes that occur in commits that are included in Pull Requests targeting your [Deploy Branch](/2.0/reference/pipelines/configurations-as-code/api#deploy_branch_name) (e.g. `main` or `master`) will trigger Terragrunt **Plan**. Changes in commits _on_ your [Deploy Branch](/2.0/reference/pipelines/configurations-as-code/api#deploy_branch_name) will trigger a Terragrunt **Apply**.
+
+The recommended workflow when working with Pipelines is to create a new Pull Request with the desired changes, then review the output of Terragrunt Plan to confirm that the resulting infrastructure changes are expected. We recommend enforcing [Branch Protection](/2.0/docs/pipelines/installation/branch-protection/#recommended-settings) and especially the [Require branches to be up to date](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches#require-status-checks-before-merging) status check on your repository as this will restrict the PR from being merged if the Plan may be out of date.
+
+## Running Plan
+
+To trigger a **Plan**, create an infrastructure change such as adding or modify a `terragrunt.hcl` unit on a new branch, then open a new Pull Request to merge this branch into your Deploy Branch. Once the Pull Request is open Pipelines will add a comment to the PR including the output of the Plan.
+
+![Screenshot of Plan Comment](/img/pipelines/guides/plan-comment.png)
+
+## Running Apply
+
+To run an **Apply**, merge your changes into the Deploy Branch. All commits including Merge commits on the Deploy Branch will trigger an apply if infrastructure changes are detected.
+
+Pipelines will add a comment to the (merged) Pull Request with the output of the Apply.
+
+## Skipping Pipelines Plan/Apply
+
+You may occasionally need to skip Pipelines on particular commits. This can be done by adding one of the [workflow skip messages](https://docs.github.com/en/actions/managing-workflow-runs-and-deployments/managing-workflow-runs/skipping-workflow-runs) such as `[no ci]` into your commit message.
+
+You can also modify the `paths-ignore` filter in `.github/workflows/pipelines.yml` within your repository to exclude an entire directory from triggering Pipelines.
+
+For example, to exclude a directory with the name `local-testing` you would modify the workflow
+```hcl title=".github/workflows/pipelines.yml"
+on:
+  push:
+    branches:
+      - main
+    paths-ignore:
+      # Workflow does not run only if ALL filepaths match the pattern. See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-excluding-paths
+      - ".github/**"
+      - "local-testing/**"
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+    paths-ignore:
+      - "local-testing/**"
+```
+
+## Destroying Infrastructure
+
+To **Destroy** infrastructure create a commit deleting the Terragrunt unit. Pipelines will detect the deletion and trigger Terragrunt to run a `plan -destroy` on pull requests or `destroy` on your Deploy Branch. Pipelines automatically checks out the previous committed version of the infrastructure so that Terragrunt can run in the (now deleted) directory.