Merge branch 'axil-gitlab-experiment-redirect' into 'master'

Reinstate deleted doc and redirect to another page

See merge request gitlab-org/gitlab!78210

doc/development/experiment_guide/experimentation.md

+---
+redirect_to: 'gitlab_experiment.md'
+remove_date: '2022-04-13'
+---
+
+This document was moved to [another location](gitlab_experiment.md).
+
+<!-- This redirect file can be deleted after <2022-04-13>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->