Integrate the new Tor Browser manual into Tor Browser

We need some logic:

1. Update the about: redirector to point manual to the new manual HTML for a given locale.
2. Add a parent and child actor to receive a custom event from the page to change the manual's locale. Store this locale in a preference.
3. Add a script maintained within the tor-browser repository to run within the page's content. See tor-browser-build#41800 (closed).

NOTE: We may want to gate this behind a feature preference so about:manual can still point to the old manual by default, whilst we work out the issues with the new manual.