DOC-3373: Add TinyMCE 8-specific llms.txt files for AI/LLM discoverability

[kemister85]

Ticket: DOC-3373

PR #2: tinymce/8 Branch (TinyMCE 8 Content)

What it does:

• Adds modules/ROOT/attachments/llms.txt (8.3KB):
  • Basic overview with key documentation links
  • Emphasizes TinyMCE 8 as the latest version
  • All URLs point to /latest/ (TinyMCE 8)
• Adds modules/ROOT/attachments/llms-full.txt (45KB):
  • Complete reference with all 395 documentation pages
  • Each page has a descriptive name
  • All API links include /apis/ path
  • CDN URLs, package commands, code examples
  • All content references TinyMCE 8

Why tinymce/8 branch: Content is TinyMCE 8-specific. Future versions (9, 10) will have their own llms.txt files with version-specific content.

Impact: Makes TinyMCE 8 documentation discoverable by AI tools. Files will be accessible at:
https://www.tiny.cloud/docs/llms.txt * https://www.tiny.cloud/docs/llms-full.txt

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.

Review:

• Documentation Team Lead has reviewed