docs: Update TOC in production to include SSL and email.

This adds a few missing entries to the TOC, which hadn't made sense
back when Zulip's ReadTheDocs didn't have the new collapsing feature.

Tweaked by tabbott to also give the SSL certificates doc an
appropriate title for its new role.

docs/production/index.rst

@@ -15,3 +15,5 @@ Zulip in Production
    security-model
    authentication-methods
    postgres
+   ssl-certificates
+   email

docs/production/ssl-certificates.md

@@ -1,3 +1,5 @@
+# SSL Certificates
+
 ### Using Let's Encrypt
 
 If you have a domain name and you've configured DNS to point to the