Restore much of the GitHub importer developer docs for rate limits

What does this MR do?

Following up !136392 (merged), puts make much of the development docs that was originally removed.

CC @wortschi @uchetta

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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