ci: replace our pullapprove setup with GitHub CODEOWNERS (#27690)

Summary of changes:
- created .github/CODEOWNERS with docs and config similar to the one in .pullapprove.yml
- updated docs
- updated .github/angular-robot.yml to not expect pullapprove status
- removed .pullapprove.yml

The primary motivations behind this change are:
- CODEOWNERS didn't exist when we introduced pullapprove
- CODEOWNERS is a functionality tightly integrated with github which results in better DX
- pullapprove v2 has been very unstable recently causing productivity loss
- pullapprove v2 has been deprecated in favor of v3, which requires and migration

PR Close #27690

.github/angular-robot.yml

@@ -81,7 +81,7 @@ merge:
     # require that the PR has reviews from all requested reviewers
     #
     # This enables us to request reviews from both eng and tech writers, or multiple eng folks, and prevents accidental merges.
-    # Rather than merging PRs with pending reviews, if all PullApprove requirements are satisfied and additional reviews are not needed pending reviewers should be removed via GitHub UI (this also leaves an audit trail behind these decisions).
+    # Rather than merging PRs with pending reviews, if all approvals are obtained and additional reviews are not needed, any pending reviewers should be removed via GitHub UI (this also leaves an audit trail behind these decisions).
     requireReviews: true,
 
     # whether the PR shouldn't have a conflict with the base branch
@@ -102,7 +102,6 @@ merge:
     # list of PR statuses that need to be successful
     requiredStatuses:
       - "continuous-integration/travis-ci/pr"
-      - "code-review/pullapprove"
       - "ci/circleci: build"
       - "ci/circleci: lint"