- Adds instructions block and relative link to Starred messages.
- Adds "Toggle starred messages counter" subheading.
- Adds "Searching for messages" as a related article.
This includes changing the URL to #settings/preferences, with a
transparent redirect so that existing links, like the one from Welcome
Bot, continue to work.
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.