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

.markdownlint.json

@@ -0,0 +1,8 @@
+{
+  "default": true,
+  "MD013": false,
+  "MD033": false,
+  "MD041": false,
+  "MD024": { "siblings_only": true },
+  "MD025": { "front_matter_title": "" }
+}