Bug 11698: Incorporate Tor Browser Manual pages into Tor Browser

This patch associates the about:manual page to a translated page that must be injected to browser/omni.ja after the build.

The content must be placed in chrome/browser/content/browser/manual/, so that is then available at chrome://browser/content/manual/ (tor-browser-build#34451 (closed)).

We preferred giving absolute freedom to the web team, rather than having to change the patch in case of changes on the documentation.

See also my comments in #11698 (closed).