Renames CSS rule that styles Zulip UI icons in the help center
so that it makes sense to use it regardless of whether we are
documenting a mobile or desktop/web feature.
- Documents the new contextually-aware button since the instructions
now differ depending on whether you're currently in a direct message
view or not.
- Simplifies the new topic instructions by combining the first two
tabs into a single web/desktop tab, with an optional step 2 to
change the destination stream.
Fixes#27159.
- Corrects documentation to reflect that typing notifications also
work when narrowing to all direct messages in the web/desktop app,
and disappear under certain conditions.
David Rosa