Add a simple website using mkdocs
Related with #97
I experimented with building the documentation in tracker-miners.git rather than tracker.git: see tracker-miners!169 (closed)
However, using the /tracker-miners project URL is confusing, most people would expect our URL to be /tracker; and it's not yet possible to do redirects using GitLab pages.
I realized it's practical to clone tracker-miners.git as part of the tracker.git pipeline and include all the website content that way, so let's do that.