Move user documentation to this repository

[snukky]

Currenlty the user documentation (https://marian-nmt.github.io/docs/) is in the repository with Marian website (https://github.com/marian-nmt/marian-nmt.github.io/blob/jekyll/docs/index.md). It would be better to have all documentation in the same repository where the code is, which hopefully will help to keep it more up to date. Sphinx supports markdown, so most of the content can be re-used.

A proposed approach for making documentation releases is first building both user + code documentation from marian-nmt/marian-dev/doc/, generating the HTML files, then copying them to the website source files in marian-nmt/marian-nmt.github.io, and finally building the website as usual with docs included. It means we will still keep a separate website repository, maybe with marian-dev as a submodule.

Ideally, we could keep the documentation consistent with the website, but that would require implementing a new theme in Sphinx (maybe it is not that difficult/time consuming?). Edited: this theme https://accentdesign.github.io/karma_sphinx_theme/ already looks quite similar to what we have on the website.

[graemenail]

Just noting that we would still need to keep marian-nmt/marian-nmt.github.io in place for the URL.

[snukky]

I updated the description to make the proposed approach clearer.