Add files to generate the offline version

We want to ship the offline manual with Tor Browser 11.5 (tpo/applications/tor-browser-build#34451 (closed) and tpo/applications/tor-browser#11698 (closed)).

The TL; DR of these tickets is that we'd like to generate a single HTML file with all the content, and show it from the about:manual page. We thought that using lektor also on our side and integrating the files in this repository is the best way to do so.

The style of the resulting page should be simpler than the current website.