Update agents.md docs

What does this MR do?

This MR uses Draft: docs: add AGENTS.md documentation (!216391) as a starting point to update the agents.md docs. I've applied most of the edits to this section from that MR and added a new section that introduces the two options and how they compare first. This is a first iteration, part of a larger plan we've outlined in this comment.

Preview link: https://docs.gitlab.com/upstream-review-mr-ee-216547/user/gitlab_duo_chat/agentic_chat/#customize-gitlab-duo-chat-in-your-ide

Note: I used Duo to help me pull in the edits from the original MR. I wanted it to pull in the changes, show me the changes in diff view for this file specifically, and then delete the changes to the other files on this branch after I'd resolved the conflicts. That's why the commit history looks a bit weird on this MR. I probably could have just cherry-picked the file but I really wanted to see the differences and apply edits one at a time. The commits will be squashed anyway when we merge this MR I ended up undoing this to fix the pipeline.

Related issues

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