docs: ignore doi.org in linkcheck, treat timeouts as non-broken

[rkingsbury]

Summary

The links tox environment runs sphinx-build -W -b linkcheck, which treats all warnings as errors. Two categories of spurious failures were identified:

1. 403 errors from DOI redirects — doi.org links redirect to publisher sites (Elsevier, Wiley, ACS, etc.) that block CI runners with 403 responses. The links themselves are valid; the publishers just reject automated requests.
2. Transient timeouts — intermittent network timeouts in CI emit a warning that -W converts to a build error, causing flaky failures unrelated to the actual content.

Changes to docs/conf.py

• Added r"https://doi\.org/.*" to linkcheck_ignore so DOI links are skipped entirely during link checking
• Set linkcheck_report_timeouts_as_broken = False so timeout events are reported as timeout status (not broken), which does not emit a warning and therefore does not trigger -W
• Set linkcheck_retries = 3 to give flaky links a few chances before reporting failure

Additional domains (e.g. ACS, Wiley, Elsevier) can be appended to linkcheck_ignore as they are identified.

This PR was authored by OpenAI Claude

[rkingsbury]

Fixes #357