Add documentation for Error Tracking

Note: This MR has been migrated to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24275

What does this MR do?

Adds documentation for Error Tracking. Epic here: &169 (closed)

New pages at:

• <review-app-url>/ce/user/project/operations/error_tracking.html

Related issues

Closes: #55581 (closed)

Features documented in this MR are added here: #55178 (closed), #55177 (closed)

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

/label ~Documentation