PYTHON-5847 Fix intermittent linkcheck failures due to rate-limiting

[blink1073]

PYTHON-5847

Changes in this PR

Updates doc/conf.py and the linkcheck CI job to reduce intermittent linkcheck failures:

• Set linkcheck_workers = 1 to serialize requests to dochub.mongodb.org and avoid rate-limiting
• Set linkcheck_timeout = 60 to give slow redirects more time before retrying
• Remove GitHub links from linkcheck_ignore — they are now checked properly instead of skipped
• Add linkcheck_anchors_ignore_for_url to skip anchor validation on GitHub URLs (anchors are rendered client-side and are unreliable to check)
• Add linkcheck_request_headers to pass a GitHub token when checking GitHub links, avoiding rate-limiting
• Pass GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} to the linkcheck workflow step so the token is available

Test Plan

Run the docs-linkcheck CI job a few times to confirm no more timeout or rate-limit failures on dochub or GitHub links.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[blink1073]

Draft until I rerun the link checker a few times

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[blink1073]

Okay, it passed 4 times in a row.