docs: link default_branch deprecation notice to github_branch_default resource

[leo-lem]

Summary

• Links the github_branch_default mention in the default_branch deprecation note (both the docs template and generated docs) to its full registry URL, so it reads clearly as a resource rather than a parameter.

Fixes #2716

Test plan

• make generatedocs regenerates docs/resources/repository.md from the template with no other diffs
• tfplugindocs validate passes

🤖 Generated with Claude Code

[github-actions]

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[Copilot]

Pull request overview

These provider review instructions are being used. This docs-only PR links the deprecated default_branch argument to its replacement resource.

Changes:

• Adds the resource link to the documentation template.
• Keeps generated documentation synchronized.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
templates/resources/repository.md.tmpl	Links to github_branch_default.
docs/resources/repository.md	Updates generated documentation.

[deiga]

Thanks for creating this!
There is a large refactor of this are coming in #3310, so I would suggest waiting until that is merged or then I could add a similar change to that PR