Add a simple website using mkdocs

Sam Thursfieldrequested to merge
sam/website into master

Related with https://gitlab.gnome.org/GNOME/tracker/issues/97

I experimented with building the documentation in tracker-miners.git rather than tracker.git: see https://gitlab.gnome.org/GNOME/tracker-miners/-/merge_requests/169

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.

Edited by Sam Thursfield

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