help: Document settings to automatically unmute/follow topics.

- 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.

help/include/automatically-follow-topics.md

@@ -0,0 +1,10 @@
+{start_tabs}
+
+{tab|desktop-web}
+
+{settings_tab|notifications}
+
+1. Under **Topic notifications**, select the desired option from the
+   **Automatically follow topics** dropdown.
+
+{end_tabs}