llms.txt for docs

[thibaudcolas]

Adoption of the llms.txt proposal for the Wagtail developer documentation and user guide, to help Wagtail users more easily find information via AI tools.

Intended outcome

Versioned llms.txt variants for docs.wagtail.org and guide.wagtail.org, refined to support different context windows. Tested with common queries for quality / suitability, so those files can be maintained long-term.

More information

• llms.txt to make the docs more accessible to LLMs #13097
• llms.txt for guide website content #412
• wagtail-prompt