[MAINT] Migrate to use `tfplugindocs` for managing docs

[deiga]

Resolves #2819 #2961 #2897 #2582

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Before the change?

• Managing docs was entirely manual and prone to forgetfulness

After the change?

• Added new tooling for generating docs from the schema (tfplugindocs)
• Updated data_source_github_app.go docs template to leverage Schema content
• Removed Organization and Repository Project docs as their resources were removed

Pull request checklist

• [ ] ~Schema migrations have been created if needed (example)~
• [ ] ~Tests for the changes have been added (for bug fixes / features)~
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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TODO:

• [ ] Integrate docs validation into GHA Workflow
• [ ] Fix reasonable Markdownlint "issues"
• [ ] Fix reported docs issues

[github-actions[bot]]

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