chore(deps): update Hugo to v0.151.0 and Docsy to v0.12.0

- Upgrade Hugo from v0.125.4 to v0.151.0+extended
- Upgrade Docsy theme from v0.10.0 to v0.12.0
- Update Bootstrap to v5.3.8
- Update Go to v1.25.1
- Add Node.js for PostCSS support
- Install PostCSS, autoprefixer dependencies
- Remove obsolete custom render-heading template
- Update minimum Hugo version requirement to 0.151.0

devbox.json

@@ -1,9 +1,11 @@
 {
   "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.10.5/.schema/devbox.schema.json",
   "packages": [
-    "hugo@0.125.4",
-    "dart-sass@1.75.0",
-    "go@latest"
+    "hugo@latest",
+    "dart-sass@latest",
+    "go@latest",
+    "nodejs@latest",
+    "htmltest@latest"
   ],
   "shell": {
     "init_hook": [],