Add temporarily visible web content-size overlay after resizing window when letterboxing is enabled

Last remaining piece of work here is to implement the web-content size status overlay

• https://www.figma.com/file/rWgMwiiFTDFp4ujuP3PKbq/Tor-Browser-13.5?type=design&node-id=4%3A12631&mode=design&t=nv1tEVOnZfe70UHy-1

Old text:

While we're trying to make the letterboxing appearance look more intentional, we may want also to let user easily access an explanation of what it is.

Possible (alternative or combined) approaches variously surfaced in parent #32324:

1. Informative tooltip when user hovers the letterboxing area
2. "?" shaped cursor, making the letterboxing area clickable to open the relevant manual section (or the options page if/when #41916 (closed) gets implemented)
3. "?" button appearing somewhere in the letterboxing area when there's enough space, instead of making the whole area clickable.

/cc @ruihildt, @thorin