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.
- Updates Desktop/web instructions to follow current help center
documentation patterns.
- Adds instructions to unsubscribe from a stream via the stream settings
panel.
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.
