Add a homepage

docs/index.md

-TODO
+# Pleroma documentation
+
+## Quick Navigation
+* [Pleroma Clients](backend/clients)
+* [Installing on Linux using OTP releases](backend/installation/otp_en.md)
+* [Configuration Cheat Sheet](backend/configuration/cheatsheet.md)
+* [Pleroma-FE configuration and customization for instance administrators](frontend/CONFIGURATION.md)
+
+## Community Channels
+* IRC: **#pleroma** and **#pleroma-dev** on freenode, webchat is available at <https://irc.pleroma.social>
+* Matrix: <https://matrix.to/#/#freenode_#pleroma:matrix.org>
+* Code: <https://git.pleroma.social/pleroma/>
+