fix(web.config): register missing MIME types for llms.txt

[dobromirts]

Currently missing llms.txt files on stagin and also images in webp format

1. Add .txt MIME type to enable IIS to serve llms.txt (and related LLM context files generated by the Astro integration)
2. Add .webp and .avif MIME types to support Astro's built-in image optimization output
   3., Applied to both docs/angular/public/web.config and docs/xplat/public/web.config