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.

.gitignore

@@ -6,5 +6,7 @@ node_modules
 .env
 app/hp_ssh.wasm
 app/wasm_exec.js
+/docs/.vitepress/dist/
+/docs/.vitepress/cache/
 
 /.direnv