Simplify broken links checking

[ckunki]

PK currently uses GitHub action gaurav-nelson/github-action-markdown-link-check in files

• ci-build.yml
• broken_links_checker.yml

Problems:

• gaurav-nelson/github-action-markdown-link-check reports Deprecation (Apr 2025) and proposes using fork tcort instead (already implemented in #676: Upgrade GitHub actions, plugins & dependencies #677)
• file broken_links_checker.yml supports to be triggered by workflow_dispatch, but ci-build.yml uses an individual step instead. This avoids redundant checkout, but duplicates the configuration of the broken links checker.