Add screenshot and features to the home page

[waldyrious]

The video in the home page is great but it only shows how to set up the bot — it doesn't show it in action.

I would have liked to see a screenshot similar to the one used in mautrix-telegram's README:

And maybe also a short summary of the features supported by the bot, like the list at https://matrix.org/docs/projects/bridge/mautrix-telegram.