Add when-to-use guidance for agents

[albinazs]

Addresses #414

Description

Reported in #485 ("No agent instruction file with when-to-use guidance found - Required")

Adds guidance to /llms.txt on when agents should use the guide.

• New "When to use", "When not to use" and "How to use" guidance before the table of contents. It covers admin and editor tasks, and sends developer questions to docs.wagtail.org.
• llms-full.txt, the agent skill and the skills index now go under an ## Optional section.
• The skill description and guardrails now include the same "not for developer questions" scope.

AI usage

AI-assisted issue investigation, code generation, and PR description.

[thibaudcolas]

great!