test: add comprehensive testing infrastructure

- Add npm test scripts for build, markdown, HTML, and link validation
- Install markdownlint-cli for content quality checks
- Install html-validate for HTML5 conformity validation
- Add htmltest (via devbox) for internal/external link checking
- Configure test rules in .htmltest.yml, .htmlvalidate.json, .markdownlint.json
- Add GitHub Actions workflow for automated CI testing
- Add TESTING.md documentation for test usage

package.json

@@ -0,0 +1,20 @@
+{
+  "name": "ipceicis-developerframework",
+  "version": "1.0.0",
+  "scripts": {
+    "build": "hugo --gc --minify",
+    "test:build": "hugo --gc --minify --logLevel info",
+    "test:links": "htmltest",
+    "test:html": "html-validate 'public/**/*.html'",
+    "test:markdown": "markdownlint 'content/**/*.md'",
+    "test": "npm run test:build && npm run test:markdown && npm run test:html && npm run test:links",
+    "test:quick": "npm run test:build && npm run test:markdown"
+  },
+  "devDependencies": {
+    "autoprefixer": "^10.4.21",
+    "html-validate": "^10.1.2",
+    "markdownlint-cli": "^0.45.0",
+    "postcss": "^8.5.6",
+    "postcss-cli": "^11.0.1"
+  }
+}