Add weekly linkchecker and report failures on issue

[melissawm]

Description

Currently, we perform a full link check (for external URLs) on every CI run, which:

• slows the documentation check,
• fails if we are trying to reference new pages in a PR, and
• can result in rate limiting issues (notably seen for some HuggingFace links).

This PR changes the link checker to only run once a week (on mondays) and to report any failures as an issue to the MaxText repo.

As a note, the internal links (those referencing documents within the MaxText docs folder) are automatically checked during the docs build. The current docs build does NOT fail on warnings (including broken internal links), but it should, and this will be fixed in a subsequent PR.

The link checker can also be run manually from the repo Actions tab.

Tests

To see what this would look like, I have implemented this fully on my fork of MaxText. You can see the action run here: https://github.com/melissawm/maxtext/actions/runs/25226702400

Note it has failed (as expected) and the generated issue with the failure report is here: melissawm#2

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• [N/A] I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.