Docs: Update files to remove Overview

doc/development/shell_scripting_guide/index.md

 # Shell scripting standards and style guidelines
 
-## Overview
-
 GitLab consists of many various services and sub-projects. The majority of
 their backend code is written in [Ruby](https://www.ruby-lang.org) and
 [Go](https://golang.org). However, some of them use shell scripts for
@@ -15,7 +13,7 @@ should be eventually harmonized with this guide. If there are any per-project
 deviations from this guide, they should be described in the
 `README.md` or `PROCESS.md` file for such a project.
 
-### Avoid using shell scripts
+## Avoid using shell scripts
 
 CAUTION: **Caution:**
 This is a must-read section.

doc/security/rack_attack.md

@@ -4,8 +4,6 @@ type: reference, howto
 
 # Rack Attack initializer
 
-## Overview
-
 [Rack Attack](https://github.com/kickstarter/rack-attack), also known as Rack::Attack, is a Ruby gem
 that is meant to protect GitLab with the ability to customize throttling and
 to block user IP addresses.

doc/user/instance/clusters/index.md

@@ -2,8 +2,6 @@
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/39840) in GitLab 11.11.
 
-## Overview
-
 Similar to [project-level](../../project/clusters/index.md)
 and [group-level](../../group/clusters/index.md) Kubernetes clusters,
 instance-level Kubernetes clusters allow you to connect a Kubernetes cluster to