Rewrite README as a short project front page; add a Nix flake

The README is now a landing page: the idea (a mesh VPN with no server, joined
with a network name and a secret), install and platform tables, exit nodes,
and a short build section. Implementation detail stays in docs/.

The flake builds tsng and the tray from source and ships a NixOS module
(services.tsunagi). Dist install notes now show the real `join -n -s` syntax,
and two links that pointed into the old README are redirected.

Co-Authored-By: Claude <noreply@anthropic.com>

dist/arch/README.md

@@ -22,7 +22,7 @@ service's socket at `/run/tsunagi/agent.sock`):
 
 ```sh
 sudo tsng status
-sudo tsng join <network-name> <tsn1…secret>
+sudo tsng join -n <network-name> -s <tsn1…secret>
 ```
 
 ## How it runs