feat(astro/config): add navigation link to Nix CN community

- Added starlight-ui-tweak plugin
- Added navigation link to Nix CN community

Signed-off-by: Noa Virellia <noa@requiem.garden>

package.json

@@ -13,7 +13,8 @@
         "@astrojs/mdx": "^4.3.12",
         "@astrojs/starlight": "^0.37.0",
         "astro": "^5.6.1",
-        "sharp": "^0.34.2"
+        "sharp": "^0.34.2",
+        "starlight-ui-tweaks": "^0.1.0"
     },
     "devDependencies": {
         "prettier": "^3.7.4",