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>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1790769526,
+        "narHash": "sha256-5AIVFLRcx8YAwbFTVPTs7YhwJH7JyHfAQsvXmpovcXI=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c9fe7d12cd78d1adcd12dd15e24432dde5b155a0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}