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 delete a topic is not yet implemented
for the initial launch of the Flutter mobile app, so we remove
the legacy React Native app instructions.
This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/1549
Part of #34748.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
