docs: Generate documentation with sphinx

[Melkor333]

I introduced sphinx docs and just used our Adfinis template (FWIW this can probably be replaced by plain sphinx+configuration at some point, see my PR and its discussion).
- Please download the latest artifact and take a look at it. I'm quite happy with how it looks right now (not perfect, but IMO good enough to be merged)

• Added a docs task in the Makefile
  • Yes I manually install md-click because it's kinda abandoned. I'd love to revive it in the Adfinis Namespace, but if you devs don't have a use for it, let's just stick to the quick-fix for now.
• usage docs with md-click. This generates multiple files which I merge to make it more convenient
  • I had to rename @cli.command to @pyaptly.command in cli.py to make the generated docs more comprehensible.
  • I merge the generated files and do some adjustment of the headers in the Makefile
• code docs with Autodocs (not very useful but only took like 5 mins... :) )
• A jsonschema of the configuration file at pyaptly/config.schema.json
  • I use it to generate the docs
  • I also added a validation function to the main.py which uses the jsonschema.
• My linter did some things in publish.py and test_publish.py which I added in a separate commit

[Melkor333]

This should also close #113 when finished.

[Melkor333]

Got internal feedback that maybe gitbook or vuebook might be a "more modern" approach than going back to sphinx... Since everything is markdown the only missing piece would be the pydocs which could probably be converted to md with something like pydoc-markdown

[winged]

Awesome work! I only have a little nitpick, otherwise LGTM!