Show the README.md file of a module instead of maintaining separate documentation pages

Currently, the dune-website repo contains a page of documentation for each module. Developers rarely look at this, and therefore the information is frequently outdated.

On the other hand, each modules contains (or should contain) a file README.md that fulfills the same purpose: Give an introduction to the module. To reduce maintainance, this merge request adds infrastructure that allows to show this file directly from the git repo instead of the page currently in dune-website. The merge request currently uses the feature only for the page for dune-functions.

There are a few open issues:

• It works for me locally, but apparently not on the server (Update: Apparently the hugo version on the server is too old.)
• Somehow the first paragraph of the included file ends up in a preformatted block
• Error handling is bad: If the file name is mistyped, then the GitLab entry page is shown, instead of an appropriate error message.

Help is appreciated.