[DOCS] Migrate documentation from VuePress 1.x to VitePress (#2299)

- Replace VuePress 1.9.10 with VitePress 1.6.4
- Convert config from CommonJS to ESM TypeScript (config.mts)
- Convert shared Theengs nav config to ESM (commonConfig.mts)
- Rewrite BoardEnvironmentTable in Vue 3 Composition API
- Rewrite FlashEnvironmentSelector in Vue 3 Composition API
- Enable built-in local search and dark mode
- Add medium-zoom for image zoom
- Use VitePress CSS variables for dark mode compatibility
- Update CI script paths from .vuepress to .vitepress
- Update .gitignore for VitePress build artifacts
- Rename README.md files to index.md (VitePress convention)
- Fix markdown compatibility (angle-bracket escaping, image paths)
- Drop PWA plugin (not needed)

Co-authored-by: Florian <1technophile@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/.vitepress/defaults.json

@@ -0,0 +1,12 @@
+{
+    "title": "Theengs OpenMQTTGateway",
+    "version": "edge",
+    "url_prefix": "/",
+    "dest": "generated/site",
+    "mode": "prod",
+    "url_license_file": "https://github.com/1technophile/OpenMQTTGateway/blob/development/LICENSE.txt",
+    "url_community_forum": "https://community.openmqttgateway.com",
+    "hostname": "https://docs.openmqttgateway.com",
+    "repo": "1technophile/OpenMQTTGateway",
+    "theme_config_repo": "1technophile/OpenMQTTGateway"
+}