docs: fix link checker errors and warnings; run weekly

[maxrjones]

Summary

This PR updates the link checker workflow to run weekly rather than daily (sorry about the noise earlier). It also fixes one error and many redirect warnings (not strictly necessary, but might as well).

For reviewers

Nothing, I'll self merge this unless there are comments in the next day or so.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)