Fix/doc linkcheck optional

[rex-schilasky]

Fix doc CI: make mdbook-linkcheck preprocessor optional

Problem

The doc CI job was failing with:

ERROR The command `mdbook-linkcheck` wasn't found, is the `linkcheck` preprocessor installed?
ERROR Unable to run the preprocessor `linkcheck`
    Caused by: No such file or directory (os error 2)

docs/book.toml declares a [preprocessor.linkcheck] section, which requires the external mdbook-linkcheck binary at build time. The CI workflow only installs mdbook — mdbook-linkcheck was never installed, causing the build to abort.

Fix

Added optional = true to the [preprocessor.linkcheck] section in docs/book.toml. This instructs mdBook to skip the preprocessor gracefully when mdbook-linkcheck is not present, rather than failing the build.

Alternatives considered

• Install mdbook-linkcheck in CI — would re-enable link checking in every build, but mdbook-linkcheck is currently unmaintained and may add fragility. Can be revisited separately.