Add entry about letterboxing (about:manual#letterboxing)

We're implementing a Learn more link in the new user-facing letteboxing preferences (tpo/applications/tor-browser#41916 (closed)) and we need some content to be referenced by about:manual#letterboxing :)

@donuts' comment:

It's on support-dot, but possibly not the manual? https://support.torproject.org/tbb/maximized-torbrowser-window/