Docs for RCA in Duo Chat

What does this MR do and why?

Updates documentation for root cause analysis in GitLab Duo Chat:

• Moves RCA topic from the experiments section to the GA section as a sub-heading for Duo Chat in GitLab Duo.
  • In the RCA topic, updates the details and adds a history item.
• Moves RCA topic from GitLab Duo experiments to Ask GitLab Duo Chat.
  • In the RCA topic, updates the prerequisites and adds details about the /troubleshoot slash command.
  • Updates the supported slash commands.
• Adds a section under See why a job failed in Jobs.

Relates to: #441681 (closed)

Resolves: #458239

Review app links:

• https://main-ee-158850.docs.gitlab-review.app/ee/ci/jobs/index.html#troubleshoot-a-failed-job-with-root-cause-analysis
• https://main-ee-158850.docs.gitlab-review.app/ee/user/gitlab_duo/#root-cause-analysis
• https://main-ee-158850.docs.gitlab-review.app/ee/user/gitlab_duo_chat/examples.html#troubleshoot-failed-cicd-jobs-with-root-cause-analysis