add quick links from user docs, thanks @gaba

doc.md

-# Documentation
+# User Documentation
 
 This documentation is primarily aimed at users.
 
+Note: most of this documentation is a little chaotic and needs to be
+merged with the [service listing](service). You might interested in
+one of the following quick links instead:
+
+ * [Email](service/email)
+ * [Forum (Discourse)](service/forum)
+ * [GitLab](howto/gitlab)
+ * [Nextcloud](service/nextcloud)
+
 <!-- update with `ls -d doc/*.md | sed 's/.md$//;s/\(.*\)/ * [\1](doc\/\1)/'` -->
 
  * [accounts](doc/accounts)