actions: Document fine-grained permissions

[anomiex]

Proposed changes:

We had documented the OAuth-style scopes where it was likely to matter, but never mentioned the newer "permissions" for GitHub Apps and fine-grained access tokens.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Fixes #40626

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Check the updated docs for accuracy and readability.

[github-actions]

Thank you for your PR!

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