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