feat: Add `gitlab-qa` as an easy way to run QA against a GitLab environment

Part of https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/2825

This change adds a GitLab CI snippet and documentation that allows
the running of GitLab QA against any GitLab instance.

gitlab-qa.md

+# [`gitlab-qa`](./gitlab-qa.yml)
+
+Runs the [GitLab QA Orchestrator](https://gitlab.com/gitlab-org/gitlab-qa) in a specified configuration,
+depending on inputs to the include.
+
+## Setup process:
+
+1. Ensure that your GitLab instance you wish to QA is available from the runner running your jobs
+
+1. Make sure you know of the root password to the GitLab instance (if it's not the default)
+
+1. Determine the QA image (that contains the test code) you wish to use for this environment. You typically want a version of the test code that matches the version of GitLab
+that is running in the environment. Some example QA images are
+
+    * `registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:16.7.3-ee`
+    * `registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:16-7-stable-ee`
+    * `registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:f7d7a15ad657694bfc7ac7b02538c648822401c7`
+
+    These images are typically built using the `build-qa-image` job in the [gitlab-org/gitlab](https://gitlab.com/gitlab-org/gitlab) project.
+
+1. Add the following to your `.gitlab-ci.yml`
+
+```yaml
+stages:
+  - gitlab-qa
+
+include:
+  # Runs gitlab-qa on an environment
+  # see https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/gitlab-qa.md
+  - project: 'gitlab-com/gl-infra/common-ci-tasks'
+    ref: v2.1.2  # renovate:managed
+    file: gitlab-qa.yml
+    inputs:
+      gitlab_initial_root_password: 'mypassword'
+      gitlab_url: 'https://mygitlab.example.com'
+      qa_image: 'registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:16.7.3-ee'
+```

gitlab-qa.yml

+spec:
+  inputs:
+    stage:
+      default: gitlab-qa
+    gitlab_initial_root_password:
+      default: "5iveL!fe"
+    gitlab_url:
+    signup_disabled:
+      default: 'false'
+    qa_image:
+    qa_test_suite:
+      default: "Test::Instance::Any"
+    timeout:
+      default: "4h"
+---
+
+include:
+  - project: 'gitlab-org/quality/pipeline-common'
+    ref: master
+    file: ci/base.gitlab-ci.yml
+
+gitlab-qa:
+  stage: $[[ inputs.stage ]]
+  extends:
+    - .qa-base
+  timeout: $[[ inputs.timeout ]]
+  variables:
+    QA_SCENARIO: $[[ inputs.qa_test_suite ]]
+    RELEASE: $[[ inputs.qa_image | expand_vars ]]
+    GITLAB_QA_OPTS: --address $[[ inputs.gitlab_url ]]
+    GITLAB_INITIAL_ROOT_PASSWORD: $[[ inputs.gitlab_initial_root_password | expand_vars ]]
+    SIGNUP_DISABLED: $[[ inputs.signup_disabled ]]