- Adds a "Topic notifications" page, just under "Stream notifications"
in the left sidebar, using Markdown include blocks for reusability
in `follow-a-topic.md` and `mute-a-topic.md`.
- Adds a tip block to clarify that disabling followed topic global
settings will not stop notifications if you already have stream
level settings enabled.
Fixes part of #27297.
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.