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.

docs/running-headscale-linux.md

@@ -0,0 +1,170 @@
+# Running headscale on Linux
+
+## Goal
+
+This documentation has the goal of showing a user how-to set up and run `headscale` on Linux.
+In additional to the "get up and running section", there is an optional [SystemD section]()
+describing how to make `headscale` run properly in a server environment.
+
+## Configure and run `headscale`
+
+1. Download the latest [`headscale` binary from GitHub's release page]():
+
+```shell
+wget --output-document=/usr/local/bin/headscale \
+   https://github.com/juanfont/headscale/releases/download/v<HEADSCALE VERSION>/headscale_<HEADSCALE VERSION>_linux_<ARCH>
+```
+
+1. Make `headscale` executable:
+
+```shell
+chmod +x /usr/local/bin/headscale
+```
+
+1. Prepare a direction to hold `headscale` configuration and the [SQlite]() database:
+
+```shell
+# Directory for configuration
+
+mkdir -p /etc/headscale
+
+# Directory for Database, and other variable data (like certificates)
+mkdir -p /var/lib/headscale
+```
+
+1. Create an empty SQlite datebase:
+
+```shell
+touch /var/lib/headscale/db.sqlite
+```
+
+1. Create a `headscale` configuration:
+
+```shell
+touch /etc/headscale/config.yaml
+```
+
+It is strongly recommended to copy and modifiy the [example configuration](../config.yaml)
+from the [headscale repository](../)
+
+1. Start the headscale server:
+
+```shell
+  headscale serve
+```
+
+This command will start `headscale` in the current terminal session.
+
+To continue the tutorial, open a new terminal and let it run in the background.
+Alternatively use terminal emulators like [tmux]() or [screen]().
+
+To run `headscale` in the background, please follow the steps in the [SystemD section]() before continuing.
+
+1. Verify `headscale` is running:
+
+Verify `headscale` is available:
+
+```shell
+curl http://127.0.0.1:8080/metrics
+```
+
+1. Create a namespace ([tailnet]()):
+
+```shell
+headscale namespaces create myfirstnamespace
+```
+
+### Register a machine (normal login)
+
+On a client machine, execute the `tailscale` login command:
+
+```shell
+tailscale up --login-server YOUR_HEADSCALE_URL
+```
+
+Register the machine:
+
+```shell
+headscale --namespace myfirstnamespace nodes register --key <YOU_+MACHINE_KEY>
+```
+
+### Register machine using a pre authenticated key
+
+Generate a key using the command line:
+
+```shell
+headscale --namespace myfirstnamespace preauthkeys create --reusable --expiration 24h
+```
+
+This will return a pre-authenticated key that can be used to connect a node to `headscale` during the `tailscale` command:
+
+```shell
+tailscale up --login-server <YOUR_HEADSCALE_URL> --authkey <YOUR_AUTH_KEY>
+```
+
+## Running `headscale` in the background with SystemD
+
+In this section it will be demonstrated how to run `headscale` as a service in the background with [SystemD]().
+This should work on most modern Linux distributions.
+
+1. Create a SystemD service configuration at `/etc/systemd/system/headscale.service` containing:
+
+```systemd
+[Unit]
+Description=headscale controller
+After=syslog.target
+After=network.target
+
+[Service]
+Type=simple
+User=headscale
+Group=headscale
+ExecStart=/usr/local/bin/headscale serve
+Restart=always
+RestartSec=5
+
+# Optional security enhancements
+NoNewPrivileges=yes
+PrivateTmp=yes
+ProtectSystem=strict
+ProtectHome=yes
+ReadWritePaths=/var/lib/headscale /var/run/headscale
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+RuntimeDirectory=headscale
+
+[Install]
+WantedBy=multi-user.target
+```
+
+1. In `/etc/headscale/config.yaml`, override the default `headscale` unix socket with a SystemD friendly path:
+
+```yaml
+unix_socket: /var/run/headscale/headscale.sock
+```
+
+1. Reload SystemD to load the new configuration file:
+
+```shell
+systemctl daemon-reload
+```
+
+1. Enable and start the new `headscale` service:
+
+```shell
+systemctl enable headscale
+systemctl start headscale
+```
+
+1. Verify the headscale service:
+
+```shell
+systemctl status headscale
+```
+
+Verify `headscale` is available:
+
+```shell
+curl http://127.0.0.1:8080/metrics
+```
+
+`headscale` will now run in the background and start at boot.