feat: documentation website using Vitepress

Adds documentation website using https://vitepress.dev.

Some decisions are questionable, for example creation of a symbolic link
for the README in the `docs/`. I wanted to preserve the ability to read
docs using Github. Maybe not needed.

Another issue, is the page outline for the `NixOS` chapter is too
narrow.

But should serve as a scaffold/basis for doing it properly.

docs/index.md

@@ -0,0 +1,22 @@
+---
+# https://vitepress.dev/reference/default-theme-home-page
+layout: home
+
+hero:
+  name: "A feature-complete Web UI for Headscale"
+  text: "Documentation"
+  tagline: "A feature-complete Web UI for Headscale"
+  actions:
+    - theme: brand
+      text: Getting Started
+      link: /README
+
+features:
+  - title: Web SSH
+    details: Connect to machines directly from your browser
+  - title: OIDC
+    details: Login through your favourite SSO provider
+  - title: And many more
+    details: Setup ACLs, DNS, pre-auth keys, etc.
+---
+