ci: Check only updated docs links in PRs

[lambdanis]

The CI job checking if links in docs are available is run both as a cron and on PRs updating docs. On PRs it checks all links, not only those added/modified in the PR. This means that if there is a known broken link, CI fails on unrelated PRs updating docs.

Example:

1. Cron job found a broken link and opened an issue: Documentation: broken links automatic report #2209
2. CI on unrelated policyfilter: Apply Policy Only to Specific Containers in a Pod #2231 failed because of this link: https://github.com/cilium/tetragon/actions/runs/8315157105/job/22754113804?pr=2231

The CI job running on PRs should fail only if one of the links added/modified in the PR is unavailable, and ignore other broken links.