Add checks for broken links

[ErikDanielsson]

The internal links to the CRBD model in my Pigeons tutorial broke due to restructuring of the website in subsequent PRs. To avoid this happening again I've added two checks:

• Docusaurus will now error when building on erroneous links instead of merely warning -- see the change in docusaurus.config.js.
• I've added a link checker, linkinator, in the gh actions that will run on each pull request. This will crawl the paths given (currently docs/**/*.md) and check all links, internal and external.
  • Since DOIs were taking a long time on my machine, as many of the sites have crawler checks supposedly, I decided to skip them. I thought that this was sensible as they supposed not to change anyway.

I have also updated internal links to use relative paths rather than absolute, and to have an explicit .md suffix. To reference a subpage, link directly to subdir/index.html rather than subdir/. This is what is recommended in the docusaurus docs from what I can tell and ensures that the crawler finds the files.