add quick links from user docs, thanks @gaba

# 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)