Replace autodoc with autoapi

[manycoding]

https://github.com/rtfd/sphinx-autoapi

1. doesn’t run the code, it just parses the files, thus removes any need from installing the package and solves dependencies overhead.
2. doesn’t require you to execute the extension each time code is updated
3. The setup is very simple.

The only cons I remember is that it uses some pre GO dependency which is not really required. I just don't like installing pre-releases.

[manycoding]

It turns out the use case is to include docstrings by manually adding such directives https://github.com/scrapinghub/spidermon/pull/165/files#diff-4786b3bb718002eab0f43f719542eaa8R139, and not creating docs for the source code. ~I am not sure if autoapi can be applicable in this case.~ Yes it can, it parses these directives.