Add Code Review Analytics documentation

This adds a documentation page for the Code Review Analytics feature,
and a link to it on the Analytics index page.

doc/user/analytics/code_review_analytics.md

+---
+description: "Learn how long your open merge requests have spent in code review, and what distinguishes the longest-running." # Up to ~200 chars long. They will be displayed in Google Search snippets. It may help to write the page intro first, and then reuse it here.
+---
+
+# Code Review Analytics **(STARTER)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/38062) in GitLab ([Starter](https://about.gitlab.com/pricing/)) 12.7.
+
+Want to learn how long your open merge requests have spent in code review?  Or what distinguishes your longest-running code reviews?  These are some of the questions Code Review Analytics is designed to answer.
+
+NOTE: **Note:**
+Initially no data will appear. Data will populate as users comment on open merge requests.
+
+## Overview
+
+Code Review Analytics displays a collection of merge requests in a table.  These are all the open merge requests that are considered to be in code review.  This feature considers code review to begin when a merge request receives its first comment from someone other than the author.  The rows of the table are sorted by review time so the longest reviews appear at the top.  There are also columns to display the author, approvers, comment count, and line -/+ counts.
+
+This feature is designed for [development team leaders](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead) and others who want to understand broad code review dynamics, and identify patterns to help explain them.  You can use Code Review Analytics to expose your team's unique challenges with code review, and identify improvements that might substantially accelerate your development cycle.
+
+## Use cases
+
+Perhaps your team agrees that code review is moving too slow, or the [Cycle Analytics feature](https://docs.gitlab.com/ee/user/analytics/cycle_analytics.html) shows that "Review" is your team's most time-consuming step.  You can use Code Review Analytics to see what is currently moving slowest, and analyze the patterns and trends between them.  Lots of comments or commits?  Maybe the code is too complex.  A particular author is involved?  Maybe more training is advisable.  Few comments and approvers?  Maybe your team is understaffed.
+
+## Permissions
+
+- On [Starter or Bronze tier](https://about.gitlab.com/pricing/) and above.
+- By users with [Reporter access] and above.
+
+## Feature flag
+
+Code Review Analytics is [currently protected by a feature flag](https://gitlab.com/gitlab-org/gitlab/issues/194165) that defaults to "enabled" - meaning the feature is available.  If you experience performance problems or otherwise wish to disable the feature, a GitLab administrator can execute a command in a Rails console:
+
+```ruby
+Feature.disable(:code_review_analytics)
+```

doc/user/analytics/index.md

@@ -15,6 +15,8 @@ Once enabled, click on **Analytics** from the top navigation bar.
 
 From the centralized analytics workspace, the following analytics are available:
 
+- [Code Review Analytics](code_review_analytics.md), enabled with the `code_review_analytics`
+  [feature flag](../../development/feature_flags/development.html#enabling-a-feature-flag-in-development). **(STARTER)**
 - [Cycle Analytics](cycle_analytics.md), enabled with the `cycle_analytics`
   [feature flag](../../development/feature_flags/development.html#enabling-a-feature-flag-in-development). **(PREMIUM)**
 - [Productivity Analytics](productivity_analytics.md), enabled with the `productivity_analytics`

ee/changelogs/unreleased/38062-code-review-analytics-documentation.yml

+---
+title: Introduce Code Review Analytics page
+merge_request: 23031
+author:
+type: added