Add documentation for Error Tracking

Note: This MR was migrated from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24157. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24157#note_130239605 for details

What does this MR do?

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

New page can be seen here: http://review-docs-docs-7jgxzi.178.62.207.141.xip.io/ce/user/project/operations/error_tracking.html

New section here: http://review-docs-docs-7jgxzi.178.62.207.141.xip.io/ce/user/project/settings/index.html#error-tracking

Related issues

Closes: #55581 (closed)

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

EE backport located here:

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

Other

• Wait until the EE backport is merged https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9096