Enable DAST runner site validation by default

- Enable feature flag by default
- Add initial documentation

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67096
EE: true

config/feature_flags/development/dast_runner_site_validation.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/331082
 milestone: '14.0'
 type: development
 group: group::dynamic analysis
-default_enabled: false
+default_enabled: true

doc/user/application_security/dast/index.md

@@ -1086,6 +1086,9 @@ follows:
 
 Both methods are equivalent in functionality. Use whichever is feasible.
 
+In [GitLab 14.2 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/324990), site profile
+validation happens in a CI job using the [GitLab Runner](../../../ci/runners/index.md).
+
 #### Create a site profile
 
 To create a site profile: