Review UX of "Tor Browser Support" link in about:preferences

Copied from #41910 (comment 2941435)

Currently, we have one link in "about:preferences" shown as

{ -brand-short-name } Support

in the bottom left corner of "about:preferences". I.e. "Firefox Support" and "Tor Browser Support".

In firefox, the link goes to https://support.mozilla.org/en-US/kb/firefox-options-preferences-and-settings. I.e. SUMO page for "about:preferences".

In tor browser, the link goes to https://support.torproject.org/tbb/. I.e. the tor browser FAQs. See #32092 (closed).

In mullvad browser, the link goes to https://mullvad.net/en/help/tag/mullvad-browser/. See mullvad-browser#177 (closed).

In addition, there is another link in "about:preferences" that goes to the same page for firefox, shown when you have an empty search result as

Sorry! There are no results in Settings for “some search term”.
Need help? Visit <a>{ -brand-short-name } Support</a>

However, I'm not sure https://support.torproject.org/tbb/ is the best redirect for these contexts. I feel like the initial decisions in the linked issues were responding to the wording "Tor Browser Support" rather than the surrounding context or the original firefox SUMO page. In particular:

1. The tor project page doesn't help with understanding "about:preferences" like firefox does. Same applies to mullvad.
2. Depending on what the user is doing, or if they haven't bootstrapped yet, "about:manual" might be more useful.

So I wonder if it might make more sense to just design something more specific that works for tor browser (and mullvad browser), rather than just hijacking an existing firefox link.

/cc @donuts for some UX input