Add GitHub actions to lint markdown files and check links

[MakisH]

This adds:

• the markdownlint action to the "Check" workflow, to check all of our markdown files for common violations.
• the markdown-link-check action to check all the links in markdown files. Since this checks too many links on GitHub, it often replies with 429 too many requests. As this mainly concerns issues and pull requests that are not expected to disappear, I consider these links as alive.

This also fixes the currently detected violations and replaces all relative links to precice.org with static links (to the non-www version).

Additional motivation: this could find wrongly-nested lists, wrong heading levels, ... Things that are easy to miss visually. Besides, this also checks links and I already found a bad one. Moreover, this tries to enforce a bit more general markdown rules. GitHub allows a few differences (e.g. no empty lines before lists), which means that other tools (e.g. pandoc) could have trouble with out files in the future.

[MakisH]

@davidscn this is finally ready for review! Do you see anything strange in the diff? Do you like the new actions?

Please don't merge just yet.

[davidscn]

markdown files look cleaner 👍
Only comment from my side: we need some instructions how to run this checks locally. Otherwise, one gets the errors when committing (:wink: ). Is there a related ubuntu package?

[MakisH]

markdown files look cleaner +1
Only comment from my side: we need some instructions how to run this checks locally. Otherwise, one gets the errors when committing (wink ). Is there a related ubuntu package?

Very good point, I documented this in CONTRIBUTING.md.