docs: Reorganize auth and migrations subsystems.

- Moves "Authentication in the development environment" from subsystems
to "development/authentication.md".
- Moves "Renumbering migrations" to a section within "Schema migrations".

templates/zerver/config_error.html

@@ -88,7 +88,7 @@
                         {% if development_environment %}
                         <p>
                             See also
-                            the <a href="https://zulip.readthedocs.io/en/latest/subsystems/auth.html#saml">SAML
+                            the <a href="https://zulip.readthedocs.io/en/latest/development/authentication.html#saml">SAML
                                     guide</a> for the development environment.
                         </p>
                         {% endif %}