* 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.
Merges the "Upgrades" section from production/maintain-secure-upgrade.md
with production/modifying-zulip.md.
Contains significant textual changes by tabbott to read more clearly.
The major changes are:
* Remove the --destroy-rebuild-database option
* Merge the new and existing self-hosted server sections
* Change the wording of the Gitter document to match the Slack one