docs: add HTML parsing guide for DOMParser

[Arsybai]

Summary

• add a new tutorial for parsing HTML with DOMParser
• explain why DOMParser, window, and document are not available as Deno globals
• show examples using both LinkeDOM and Deno DOM
• add the tutorial to the Examples catalog under Web standard APIs

Why

This addresses the DOMParser / HTML parsing documentation gap tracked in #3202. The guide includes the main search terms users are looking for and provides practical alternatives for parsing HTML in Deno.

Testing

• ran deno fmt on the changed files
• verified the examples catalog entry points to the new tutorial
• verified the final diff only contains the tutorial page and catalog entry

Addresses the DOMParser / HTML parsing item in #3202.