[POC] Markdown support

[JoergAtGithub]

The example-setups page in this PR is generated from MarkDown syntax instead of RST syntax: https://deploy-preview-712--mixxx-manual.netlify.app/chapters/example_setups

There are many warnings about cross-links, but the links seem to work nethertheless.

[daschuer]

This is a nice summary:
https://nikhilakki.in/markdown-vs-restructured-text

[daschuer]

What is the status here? Is this only about getting cross link check working?

[acolombier]

Looks like the links are actually working, only seems to get this false positive on the CI, perhaps some settings to tweak?

[JoergAtGithub]

What is the status here? Is this only about getting cross link check working?

This was a prove of concept how to use MarkDown as manual source for mapping pages instead of RST.
For the PoC I selected a page with many different elements and cross links from and to it. This is more than a mapping page will have (usually only links to other manual page but not in the opposite direction). As you can see, most RST formattings could be translated to MD syntax, but not all. The links work, but there I warnings, which I don't understood.

The idea in our monthly call was to store the MD files next to the controller mapping files, that the contributor needs only one PR to one single Repo. This MD file would than be syncronized to the manual repo, using the same process that we've already to get the changelog into the manual.