docs: update GitHub App env-var auth guidance for v6.13.0 blockless behavior

[SebastianBienert]

Resolves #3550

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Before the change?

• The GitHub App Installation docs state that an empty app_auth {} block is required when authenticating through the GITHUB_APP_XXX environment variables, and the main.tf example shows that pattern.
• Since feat: Add new gh client implementation #3448 this is obsolete: getAppAuth() reads the environment variables directly, before consulting the block.
• The documented pattern also fails terraform validate / tofu validate in any environment where the variables are not set (CI linting, local pre-commit hooks), because the app_auth arguments are schema-required with EnvDefaultFunc. Details and reproduction in [DOCS]: Empty app_auth {} recommendation fails validate without credentials #3550.

After the change?

• The note states that no app_auth block is needed for environment-variable auth, and that the previously documented empty block should be removed because it fails validate without the variables set.
• The main.tf example no longer shows an empty app_auth {} block.
• No per-version guidance included: registry docs are versioned, users on older provider versions see the matching older docs.

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• 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
• No

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[github-actions]

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[deiga]

Please note that docs files are generated. You need to update the template files as well

[stevehipwell]

I'm not sure I fully agree with these changes. The docs do need to be updated to show that all of the app auth config can be driven from environment variables and the app_auth block is not required. However setting the app_auth block if you explicitly want to use app auth is the recommended pattern as it errors if the env variables haven't been set, as is using the block for static configuration (e.g. app ID).

[SebastianBienert]

Thanks @deiga, updated. Pushed the same change to the sources: templates/index.md.tmpl for the note, and examples/provider/app_auth_env/main.tf for the example, which the template inlines via {{ tffile }}. Ran go generate ./... locally to confirm docs/index.md regenerates identically, so generation stays idempotent.

[SebastianBienert]

@stevehipwell reworked to address this. The note now states the app_auth block is optional (env vars are read directly) and recommends keeping the block when you want GitHub App auth deliberately, since its required arguments make terraform validate fail fast on a missing value. Three examples now cover: block with values, environment-variables-only (no block), and non-secret IDs in config with the PEM kept in the environment.

[stevehipwell]

Thanks for the quick response @SebastianBienert.

LGTM

[stevehipwell]

@SebastianBienert could you please rebase this PR?

[SebastianBienert]

@stevehipwell done