[d-v-b]

claude wrote this at my behest! it fixes some broken links in documentation material flagged in #4291, and updates our link checker to anticipate a 403 from stackoverflow. It also updates some html link elements to use markdown instead, which plays nicer with mkdocs and then lychee.

Summary

Convert the raw-HTML prev/next nav links in the zarr-indexing guide to markdown inside <nav markdown> blocks, so mkdocs validates and rewrites them at build time and lychee resolves them as files; the rendered HTML is unchanged. Update the MDN CORS link in the zarr-http-server guide to its post-redirect URL, and exclude the bot-blocked Stack Overflow tag page from lychee.

Assisted-by: ClaudeCode:claude-fable-5

[Describe what this PR changes and why, in your own words.]

For reviewers

[What would you most value a second look at? What are you already confident in? For a refactor, say whether behavior is meant to be unchanged.]

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)

[d-v-b]

this is a non-behavioral change scoped to our link checker and the docs for two subpackages, so I'm going to merge this as soon as checks pass.