ci/docs: re-enable mdbook-linkcheck once upstream catches up

[P4suta]

Background

The mdbook link checker is disabled in crates/afm-book/book.toml:26-31:

# [output.linkcheck] is disabled here because mdbook-linkcheck upstream has
# not yet caught up to the RenderContext schema change in recent mdbook
# releases. Re-enable once compatible versions align.
# [output.linkcheck]
# follow-web-links = false
# warning-policy   = "error"

Until then, broken intra-book links and dead references are not caught by
CI.

Trigger to act

mdbook-linkcheck ships a release compatible with the mdbook version
pinned in the Docker image / book.toml [rust] edition setup (i.e. the
RenderContext schema mismatch is resolved upstream).

What to do

1. Confirm a compatible mdbook + mdbook-linkcheck pair.
2. Uncomment the [output.linkcheck] block (keep
   warning-policy = "error" so dead links fail the build).
3. Make sure the linkcheck step runs in the book stage of just ci /
   the docs workflow, and that the tool is installed in the Docker image
   that runs it.

References

• crates/afm-book/book.toml:26-31
• .github/workflows/docs.yml (docs/book build)
• Justfile book / ci recipes