Rewrite main documentation

This commit starts restructuring the documentation and updating it to be
compliant with 0.12.x+ releases.

The main change is that the documentation has been rewritten for the
ground up, and hopefully simplified.

The documentation has been split into an official documentation for
running headscale as a binary under Linux with SystemD and a "community"
provided documentation for Docker.

This should make the two documents a lot easier to read and follow than
the mishmash document we had.

config-example.yaml

@@ -8,7 +8,7 @@ listen_addr: 0.0.0.0:8080
 
 # Private key file which will be
 # autogenerated if it's missing
-private_key_path: private.key
+private_key_path: /var/lib/headscale/private.key
 
 derp:
   # List of externally available DERP maps encoded in JSON
@@ -16,8 +16,8 @@ derp:
     - https://controlplane.tailscale.com/derpmap/default
 
   # Locally available DERP map files encoded in YAML
-  paths:
-    - derp-example.yaml
+  # paths:
+  #   - /etc/headscale/derp-example.yaml
 
   # If enabled, a worker will be set up to periodically
   # refresh the given sources and update the derpmap
@@ -33,7 +33,7 @@ ephemeral_node_inactivity_timeout: 30m
 
 # SQLite config
 db_type: sqlite3
-db_path: db.sqlite
+db_path: /var/lib/headscale/db.sqlite
 
 # # Postgres config
 # db_type: postgres
@@ -48,7 +48,7 @@ acme_email: ""
 
 tls_letsencrypt_hostname: ""
 tls_letsencrypt_listen: ":http"
-tls_letsencrypt_cache_dir: ".cache"
+tls_letsencrypt_cache_dir: /var/lib/headscale/cache
 tls_letsencrypt_challenge_type: HTTP-01
 
 tls_cert_path: ""