Update UI links to moved docs pages/sections

What does this MR do?

Related to #328203 (comment 558874000)

The test that makes sure docs links are up to date missed a few edge cases. This is being worked on in the related issue, but we have to fix the links before we can fix the test (to avoid broken pipelines).

The following warnings were fixed to match the updated locations (MRs linked for verification):

• app/views/clusters/clusters/show.html.haml:9 [W] DocumentationLinks: anchor (getting-the-external-endpoint) is missing in: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/user/project/clusters/index.md

  Changed in: !18949 (diffs)

• app/views/clusters/clusters/show.html.haml:9 [W] DocumentationLinks: anchor (defining-environments) is missing in: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/ci/environments/index.md

  Changed in: !55432 (diffs)

• app/views/clusters/clusters/show.html.haml:9 [W] DocumentationLinks: anchor (cloud-run-for-anthos) is missing in: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/user/project/clusters/add_remove_clusters.md

  Changed in: !27748 (diffs)

• app/views/projects/feature_flags/edit.html.haml:7 [W] DocumentationLinks: help_page_path points to the unknown location: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/ci/environments.md
app/views/projects/feature_flags/new.html.haml:6 [W] DocumentationLinks: help_page_path points to the unknown location: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/ci/environments.md

  Changed in: !32246 (diffs)

• app/views/projects/registry/repositories/index.html.haml:6 [W] DocumentationLinks: anchor (expiration-policy) is missing in: /Users/vasya/projects/gitlab-development-kit/gitlab/doc/user/packages/container_registry/index.md

  Changed in: !34014 (diffs)

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• Documentation (if required)
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• Database guides
• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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