For mobile features that were documented and implemented for the legacy
React Native app, adds back a tabbed block for mobile with a note to
use the web app instructions in a mobile device browser, and with a
link to the Flutter GitHub issues tracking implementing the feature in
the new mobile app.
If the feature is a mobile only feature, then adds the link to the
Flutter GitHub issue in a warning note.
The ability to unsubscribe from a channel is not yet implemented
for the initial launch of the Flutter mobile app, so we remove
the legacy React Native app instructions.
Deletes static/images/help/mobile-check-icon.svg as it is no
longer used.
This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/1224
Part of #34748.
