Conversion of Sphinx based tutorial to markdown/mkdocs

[neteler]

This PR

• adds the .rst files converted with pandoc to MD into the docs/ subdir
• adds mkdocs.yml (the site style may be updated later; default for now)
• contains manual code style fixes after pandoc conversion

Can be locally build with

mkdocs build

The static site is then written to site/ with index.html etc.

Fixes #181

TODO

• verify content, layout, and update accordingly

Example

[marcjansen]

Looking good so far.

[mmacata]

This is great! Please also replace the command in the Makefile with mkdocs build
Then fine for me to be merged.