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/README.md

@@ -1,7 +1,28 @@
-# Official headscale documentation
+# headscale documentation
 
-- [Configuration](Configuration.md)
-- [Running](Running.md)
+This page contains the official and community contributed documentation for `headscale`.
+
+If you are having trouble with following the documentation or get unexpected results,
+please ask on [Discord](https://discord.gg/XcQxk2VHjx) instead of opening an Issue.
+
+## Official documentation
+
+### How-to
+
+- [Running headscale on Linux](running-headscale-linux.md)
 - [DNS](DNS.md)
 - [TLS](TLS.md)
+
+### References
+
+- [Configuration](Configuration.md)
 - [Glossary](Glossary.md)
+
+## Community documentation
+
+Community documentation is not actively maintained by the headscale authors and is
+written by community members. It is _not_ verified by `headscale` developers.
+
+**It might be outdated and it might miss necessary steps**.
+
+- [Running headscale in container](running-headscale-container.md)