Move some documentation to docs/ and start overhaul

docs/installation/prerequisites.md

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+# Prerequisites for running Conduit
+
+You'll need:
+
+- A domain. Commonly cost about $10/year.
+- A Linux server with a stable internet connection, at least 500 MB of RAM and some disk space for messages and
+  attachments. Commonly start at $5/month.
+- Some basic knowledge about using a shell, SSH and configuring and protecting a server.
+  
+  
+## A word of caution:
+
+Don't underestimate the toll of administrating your own server.
+Conduit can't protect your conversations if your server gets compromised or deleted.
+
+Make sure that you got:
+
+- Automatic security updates
+    - On Ubuntu/Debian: Set up unattended-upgrades
+    - On RHEL/CentOS: Have a look at yum-cron
+- A firewall blocking all but the needed incoming ports
+    - ufw is an easy interface for the linux firewall
+- Protection against automatic attacks
+    - fail2ban scans logs and bans IPs which try to brute force their way into your server.
+    - Disable ssh login for root and switch from password to key based authentication.
+- Automated backups
+    - Most VPS hosting companies offer whole server backups for a small fee.
+    - Or run your own backup with something like borg.
+- A way to get notified if your disk fills up.
+    - If you send too much cat videos to your friends, Conduit might at some point become unable to
+      store any important messages.