Docs: Move chatops docs from EE to core

What does this MR do?

Moves docs from EE to core for ChatOps. Chatops used to be part of Ultimate tier. No changes from EE here except adding when this feature was moved to CE.

Should be released with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24780

Related issues

https://gitlab.com/gitlab-org/gitlab-ce/issues/53243

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards