Tor Browser should open documentation links from local website when offline

Context

Right now, we have a nice tails-documentation command which can, when appropriate, convert a link like https://tails.net/doc/persistent_storage/ into a path like `/usr/share/doc/tails/website/doc/persistent_storage.en.html

problem

This is good. However, we need to explicitly call this. This means:

• we need to write more code into our GTK applications in order to make it work
• we sometimes forget to write this code
• zenity dialogs just don't support it

For example, !1332 (merged) is showing a zenity dialog just after the greeter. It has a link, but there are little chances the user could already be online, so I expect that most of the times this will give a bad UX to the user: they need to click on the link again after they managed to connect to Tor.

proposal

We could just bake the logic of tails-documentation into /usr/local/bin/tor-browser itself. This would allow this to Just Work even for users which are currently offline.

I could imagine scenarios where a user has some link, maybe in a document in their Persistent Storage, this document has a link to the website, the user clicks on this link while offline, the link opens just fine now but is not updated, and maybe they would have preferred an error compared to an outdated document. I can't find any practical usecase where this would be worse than the current behavior, though.