[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/using-github-cli-extensions.md

@@ -0,0 +1,66 @@
+---
+title: Using GitHub CLI extensions
+intro: 'Learn how to use custom extensions written by other {% data variables.product.prodname_cli %} users.'
+versions:
+  fpt: '*'
+  ghes: '*'
+  ghae: '*'
+topics:
+  - CLI
+---
+
+## About {% data variables.product.prodname_cli %} extensions
+
+{% note %}
+
+**Note:** Extensions outside of {% data variables.product.product_name %} and {% data variables.product.prodname_cli %} are not certified by {% data variables.product.product_name %} and are governed by separate terms of service, privacy policy, and support documentation. To mitigate risk when using third-party extensions, audit the source code of the extension before installing or updating the extension.
+
+{% endnote %}
+
+{% data reusables.cli.cli-extensions %} For more information about how to create {% data variables.product.prodname_cli %} extensions, see "[Creating {% data variables.product.prodname_cli %} extensions](/github-cli/github-cli/creating-github-cli-extensions)."
+
+Extensions are locally installed and are scoped to the user. Therefore, if you access {% data variables.product.prodname_cli %} from a different machine or another user accesses {% data variables.product.prodname_cli %} from the same machine, the extension will not be available.
+
+## Finding extensions
+
+You can find extensions by browsing [repositories with the `gh-extension` topic](https://github.com/topics/gh-extension).
+
+## Installing extensions
+
+To install an extension, use the `extensions install` subcommand. Replace the `owner/repo` parameter with the name of the extension, such as `octocat/gh-whoami`.{% ifversion ghes %} If the extension is on {% data variables.product.prodname_ghe_server %}, also include the hostname, such as `https://ghe.io/octocat/gh-whoami`.{% endif %}
+
+```shell
+gh extension install <em>owner/repo</em>
+```
+
+If you already have an extension by the same name installed, the command will fail. For example, if you have installed `octocat/gh-whoami`, you must uninstall it before installing `hubot/gh-whoami`.
+
+## Viewing installed extensions
+
+To view all installed extensions, use the `extensions list` subcommand. The output will also tell you which extensions have updates available.
+
+```shell
+gh extension list
+```
+
+## Updating extensions
+
+To update an extension, use the `extensions upgrade` subcommand. Replace the `extension` parameter with the name of the extension.
+
+```shell
+gh extension upgrade <em>extension</em>
+```
+
+To update all installed extensions, use the `--all` flag.
+
+```shell
+gh extension upgrade --all
+```
+
+## Uninstalling extensions
+
+To uninstall an extension, use the `extensions remove` subcommand. Replace the `extension` parameter with the name of the extension.
+
+```shell
+gh extension remove <em>extension</em>
+```