Add merge request template rule to AGENTS.md

What does this MR do?

Adds a "Merge request conventions" section to AGENTS.md instructing AI agents to read .gitlab/merge_request_templates/ before creating any MR and use the matching template (Documentation.md for docs-only changes, Default.md otherwise), filling in its sections and keeping its quick actions.

Why was this MR needed?

MR templates are only applied automatically in the web UI, not to MRs created through the API. An AI agent recently created a docs MR without the Documentation.md template and had to be corrected manually. Since AGENTS.md is injected into every agent session on this project, codifying the rule here prevents the same mistake for all future agent-created MRs.

What's the best way to test this MR?

Review the added section for clarity. Optionally, ask GitLab Duo (or another agent that consumes AGENTS.md) to create a test MR and verify it applies the correct template.

What are the relevant issue numbers?

None.