ci: Add pre-commit hook and link checker

[f-hollow]

This PR adds a pre-commit hook and a link checker as part of the hook.

Tried the following packages for link checking (see a nice overview)

• markdown-link-check
  • + Installed automatically by pre-commit
  • + No dependencies
  • -- Cannot check anchors/fragments, which is critical!
• lychee
  • - Installed either automatically via Docker or manually via cargo (takes long); both are inconvenient but Docker is faster and safer
  • - Depends on Docker
  • ++ Can check anchors/fragments

Pre-commit hook is offered as optional. But the lychee GitHub action makes sure that broken links don't get deployed.

We will be adding more pre-commit hooks in the future.

[f-hollow]

The review has already been done internally. This PR already looks good.