Feat: check links upon rebuilds of `next`

[almereyda]

This adds logic to check internal and external links upon rebuilding the next environment.

It reifies these Pull Requests that were tested on next here on main:

• https://github.com/degrowth/idn-website/pull/9
• https://github.com/degrowth/idn-website/pull/10
• https://github.com/degrowth/idn-website/pull/11
• https://github.com/degrowth/idn-website/pull/12

It also adds documentation for advanced use cases of using the link checker locally.

See the newly introduced docs/ subdirectory for details.

Check these example runs for the job summaries:

• fix(CI): caching + job names · degrowth/idn-website@6059c6c
• fix(CI): use only positive path rule for next on push trigger · degrowth/idn-website@2b2db50