[Aug 24, 2021] Top level CLI docs set (#20628)

Co-authored-by: Ethan Palm <56270045+ethanpalm@users.noreply.github.com>
Co-authored-by: Mislav Marohnić <mislav@github.com>
Co-authored-by: Laura Coursen <lecoursen@github.com>

content/github-cli/github-cli/index.md

@@ -0,0 +1,15 @@
+---
+title: GitHub CLI
+shortTitle: GitHub CLI
+intro: '{% data reusables.cli.about-cli %}'
+versions:
+  fpt: '*'
+  ghes: '*'
+  ghae: '*'
+children:
+  - /about-github-cli
+  - /quickstart
+  - /creating-github-cli-extensions
+  - /using-github-cli-extensions
+  - /github-cli-reference
+---