Draft: docs: Add documentation for AGENTS.md

What does this MR do and why?

Adds documentation for the AGENTS.md functionality added in gitlab-org/editor-extensions/gitlab-lsp!2597 (merged)

Preview link: https://docs.gitlab.com/upstream-review-mr-ee-214888/user/gitlab_duo_chat/agentic_chat/#create-agentsmd-instruction-files

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.