Enable empty section CTA in pipeline editor

This enable the possibility to work on an uncommited CI config
file in the pipeline editor section, whether you have a repo or not yet.

changelogs/unreleased/ff-enable-pipeline-editor-empty-state.yml

+---
+title: Add empty state CTA in pipeline editor section for new root CI files
+merge_request: 56665
+author:
+type: changed

config/feature_flags/development/pipeline_editor_empty_state_action.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/323229
 milestone: '13.10'
 type: development
 group: group::pipeline authoring
-default_enabled: false
+default_enabled: true

doc/ci/pipeline_editor/index.md

@@ -10,11 +10,9 @@ type: reference
 > - [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/4540) in GitLab 13.8.
 > - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/270059) in GitLab 13.10.
 
-WARNING:
-This feature might not be available to you. Check the **version history** note above for details.
-
 The pipeline editor is the primary place to edit the GitLab CI/CD configuration in
-your `.gitlab-ci.yml` file. To access it, go to **CI/CD > Editor**.
+The pipeline editor is the primary place to edit the GitLab CI/CD configuration in
+the `.gitlab-ci.yml` file in the root of your repository. To access the editor, go to **CI/CD > Editor**.
 
 From the pipeline editor page you can:
 
@@ -25,8 +23,7 @@ From the pipeline editor page you can:
 - View an [expanded](#view-expanded-configuration) version of your configuration.
 - [Commit](#commit-changes-to-ci-configuration) the changes to a specific branch.
 
-NOTE:
-You must already have [a `.gitlab-ci.yml` file](../quick_start/index.md#create-a-gitlab-ciyml-file)
+In GitLab 13.9 and earlier, you must already have [a `.gitlab-ci.yml` file](../quick_start/index.md#create-a-gitlab-ciyml-file)
 on the default branch of your project to use the editor.
 
 ## Validate CI configuration