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

.htmlvalidate.json

@@ -0,0 +1,12 @@
+{
+  "extends": ["html-validate:recommended"],
+  "rules": {
+    "no-inline-style": "off",
+    "require-sri": "off",
+    "no-trailing-whitespace": "off",
+    "void-style": "off"
+  },
+  "elements": [
+    "html5"
+  ]
+}