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.
- Adds desktop/web instructions.
- Adds #inbox relative link for logged-in users.
- Moves Inbox up in the left sidebar just under "Reading strategies".
- Moves Inbox article content to Markdown include.
- Adds "From the Inbox view" section to "Finding a topic to read",
"Getting started with Zulip", and "Reading strategies".
- Documents Inbox as a new option for the default web app view.
- Removes unused Markdown link.
- Tweaks subheading to better match help center patterns.
- Add Inbox option in "Configure default settings for new users".
- Adds new tabbed section and instructions for marking messages as
read and reading topics via the Inbox view.
Fixes#26903.
Co-authored-by: Alya Abbott <alya@zulip.com>
David Rosa