Use the `support-page` instead of `tor-manual-page` in `moz-support-link`

In #42583 (closed) we added the tor-manual-page attribute to about:manual to point a "Learn more" link to about:manual (or the Tor Project web page in base-browser).

However, since we want to use setting-group elements for our own settings, we need to be able to specify the manual reference using the config. Rather than add a new property to the setting config, which would require a fair bit of fixing up, we can instead hijack the existing supportPage property which sets the support-page attribute that upstream uses for moz-support-link.

My initial idea is that we can use a special non-conflicting prefix for the support-page attribute value that will distinguish our pages from upstream's pages.