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/Integrated-Mode.md

@@ -3,15 +3,15 @@
 <picture>
     <source
         media="(prefers-color-scheme: dark)"
-        srcset="../assets/dns-dark.png"
+        srcset="./assets/dns-dark.png"
     >
     <source
         media="(prefers-color-scheme: light)"
-        srcset="../assets/dns-light.png"
+        srcset="./assets/dns-light.png"
     >
     <img
         alt="Integration Preview"
-        src="../assets/dns-dark.png"
+        src="./assets/dns-dark.png"
     >
 </picture>
 
@@ -21,7 +21,7 @@ deployment method for most users, as it provides a more feature-complete
 experience.
 
 ## Deployment
-> If you are not looking to deploy with Docker, follow the [**Bare-Metal**](/docs/Bare-Metal.md) deployment guide.
+> If you are not looking to deploy with Docker, follow the [**Bare-Metal**](./Bare-Metal.md) deployment guide.
 > Refer to the `Integrated Mode` section at the bottom for caveats.
 
 Requirements:
@@ -73,7 +73,7 @@ This will result in the Headplane UI being available at the `/admin` path of the
 server you deployed it on. The `/admin` path is currently not configurable unless
 you build the container yourself or run Headplane in Bare-Metal mode.
 
-> Refer to the [**Configuration**](/docs/Configuration.md) guide for help with
+> Refer to the [**Configuration**](./Configuration.md) guide for help with
 > setting up your `config.yaml` file to the appropriate values.
 
 ## Docker Integration