DOC-3471, DOC-3476: Update LLM files with Markdown endpoints and Context7 MCP instructions

Rewrite all doc page URLs in llms.txt (~32) and llms-full.txt (~473)
from trailing-slash pattern to index.md endpoints, ensuring LLMs
receive clean Markdown instead of HTML when consuming TinyMCE docs.

Add Context7 MCP setup section to llms.txt with Cursor and Claude
Code configuration instructions for live documentation lookups.

Author: kemister85