Docs link check validates PR docs against the live site

[CoderMungan]

The Docs Link Check workflow currently validates generated docs in pull requests against https://spike.ist instead of the checked-out PR artifact.

In #287, lychee reported 1307 errors, mostly repeated 404s for pages such as:

• https://spike.ist/recipes/backup-and-restore/
• https://spike.ist/recipes/bootstrapping-spike/
• https://spike.ist/recipes/writing-access-policies/

Those pages existed in the PR's generated docs/ tree, but they were not published on the live site yet. Because the workflow runs before deployment, checking https://spike.ist produces false positives for newly added or changed docs pages.

Root cause:

• .github/workflows/docs-link-check.yaml runs lychee with --base-url https://spike.ist.
• .lychee.toml remaps root/file URLs back to https://spike.ist/.
• Lychee's --base-url resolves relative links; it does not make already-absolute https://spike.ist/... links resolve to local files.
• (cached) entries in the report came from lychee cache reuse, but cache only amplified the visible failure; it did not cause the wrong target selection.

Expected behavior:
For PRs, lychee should validate internal site links against the checked-out/generated docs/ tree, not the currently deployed production site.

cc @v0lkan