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 view and subscribe to channels 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-plus-icon.svg as it is no
longer used.
This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/188
Part of #34748.
