Include Tor Browser Manual in packages during build

In tor-browser#11698 (closed) we are discussing of what we need to bundle an offline manual.

My proposal is to limit the changes on tor-browser, and inject the "compiled" manual during the build.

The rationale behind this is it's a simple solution, Tor Browser has its own pace for changes, and the manual has another one, they have different requirements, not doubling the efforts is always good, especially with localization, etc...

The main disadvantage I see is that injecting contents into omni.ja is a bit hackish. (It also make creating dev builds outside tor-browser-build more difficult, but contrarily to NoScript/HTTPS-E, the manual isn't a funding TBB part).

So, for this issue, first we need to setup a new project to build the manual in all languages.

We discussed about using the same tools used for the website (e.g., lektor), but with a slightly different configuration.

In particular, since the URLs of omni.ja contents are quite ugly/really not meaningful to the users, we'd like to serve the manual through an about: page.

However, I think that associating multiple HTML files with a single about page could be involved (I'm not sure how slashes are handled by nsAboutRedirector). So, since lektor can create single page sites, we should investigate about such configuration.

Stylesheets, images and any scripts will need to have the correct chrome:// URL.

The second part of this issue is modifying the tor-browser project to inject the manual in the omni.ja.