Convert documentation to mkdocs-material

[FlorianLudwig]

Writing markdown comes a lot easier to me (and it seems others as well) then ReStructuredText. So I suggest converting the docs to markdown and use mkdocs to generate nice looking html docs.

[FlorianLudwig]

@HerrMuellerluedenscheid do you have any objections?

[HerrMuellerluedenscheid]

I also find markdown easier to write. What I don't like about markdown is that it does not support literal includes of other files. That means that examples have to be writen inside an .md file. That means that examples cannot be integrated in CI pipelines which makes puts them at greater risk to become outdated. I mean we don't check examples right now anyway, so it wouldn't change a thing. I'm also not sure though if mkdocs has a way to allow includes of other files inside markdown.
So, I'm ok with changing to markdown. Just wanted to share my thoughts :)

[FlorianLudwig]

Including is possible, https://squidfunk.github.io/mkdocs-material/reference/code-blocks/#embedding-external-files

using ASCII scissors cutting a line followed by the name of the file:

--8<-- "filename.ext"