We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
* Moves "Management commands" to a top-level section.
* Moves "Scalability" as a subsection at the bottom of "Requirements".
* Moves "Monitoring" as a subsections at the bottom of "Troubleshooting".
* Replaces "API and your Zulip URL" with a link to REST API docs. This
documentation text has been irrelevant for some time.
* Removes maintain-secure-upgrade from the TOC but the file remains to
avoid breaking old links from release blog posts and emails.
Tim Abbott
Steve Howell
Mateusz Mandera
Tim Abbott
David Rosa