Make link checker quicker

[sverhoeven]

It takes about a 1 minute to perform the check on all links in this repository and on generated repo.

The action has an option to only check changed files (check-modified-files-only).

Could we split the workflow into

1. markdown-link-check-pr.yml to only check links of changed files of PR commit
2. markdown-link-check-main.yml to check all links when PR is merged into main
3. markdown-link-check-manual.yml could a manual trigger that runs on supplied branch on all files

[egpbos]

I would just let it check everything. 1 minute is not great, but considering that most Python build workflows take more than a minute anyway due to pip installing, it doesn't matter that much, because you'll be waiting for the other workflows as well.