Letterboxing: add a screenshot to show this feature

Some users are confused about the letterboxing feature. We already have an entry in the Support portal, but we could add a screenshot pointing the borders. Tails have a screenshot, but we cannot use that image because their Tor Browser is different from ours.

Task:

1. Take a screenshot of Tor Browser
2. Edit and show letterboxing feature, for example, like Tails screenshot. Save it as letterboxing.jpg.
3. Fork support repository.
4. Add letterboxing.jpg to assets/static/images and change this entry: http://rzuwtpc4wb3xdzrj3yeajsvm3fkq4vbeubm2tdxaqruzzzgs5dwemlad.onion/tbb/maximized-torbrowser-window/
5. Send a Pull Request, one clean commit.