[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/upload/board-selector.md

@@ -1,10 +1,3 @@
---- 
-pageClass: table-generated-page
----
-
-
-<FlashEnvironmentSelector 
+<FlashEnvironmentSelector
   boardsUrl="/boards-info.json"
   firmwareBaseUrl="firmware_build/" />
-
-  

docs/upload/web-install.md

@@ -1,5 +1,5 @@
---- 
-pageClass: table-generated-page
+---
+aside: false
 ---
 
 ## Select your firmware
@@ -32,4 +32,3 @@ Support open-source development through sponsorship and gain exclusive access to
     <iframe src="https://github.com/sponsors/theengs/button" title="Sponsor Theengs" height="32" width="228" style="border: 0; border-radius: 6px;"></iframe>
 </div>
 
-