help-docs: Extend notification bot documentation.
help-docs: Extend notification bot documentation.

Updates the documentation for configuring the notification bot to
include information about the non-configurable messages sent for
stream settings / permissions changes as well as topic resolve
events.

Adds more detailed sections / information about the configurable
aspects of the notification bot, and information about the topics
for the various messages sent by the notification bot.

Fixes #21947.

andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip