* ci: add external link check (lychee, scheduled + on-demand)
Adds a non-blocking external-link checker mirroring wickra-docs: an
authoritative weekly + on-demand lychee run (this workflow), kept off the
PR/release path so a flaky third-party site can never block a merge or an
irreversible publish. Fixes the one redirecting link it would flag
(docs.rs/wickra -> docs.rs/wickra/latest/wickra/ in SUPPORT.md).
* ci: run lychee as a non-blocking PR job (continue-on-error)
Hybrid link-check: the authoritative run is the scheduled links.yml; this
adds an early heads-up on PRs that never gates the merge. release.yml stays
ungated so a flaky third-party site can never block an irreversible publish.
kingchenc