docs(forms): add link to JSON Schema

[jamietanna]

Why:

So folks can get IDE autocomplete, and use other validation tools for it.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on the review server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-githubs-form-schema.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

[Sharra-writes]

Thanks so much for opening a PR! I'll get this triaged for review.

[Sharra-writes]

Thank you for opening a PR! Per our “About contributing to GitHub Docs” article -

We only document GitHub products, features, tools, and extensions. We may mention or link to third-party tools to demonstrate how a feature works, but we do not accept pull requests to document third-party tools or integrations unless they were codeveloped with GitHub.

For this reason, we will be closing this PR. If you would like to update our docs with content outside of third-party tools or integrations, feel free to open another PR or issue 💛