gh-extension: add page (#6407)

pages/common/gh-extension.md

@@ -0,0 +1,24 @@
+# gh extension
+
+> Manage extensions for the GitHub CLI.
+> More information: <https://cli.github.com/manual/gh_extension>.
+
+- Initialize a new GitHub CLI extension project in a directory of the same name:
+
+`gh extension create {{name}}`
+
+- Install an extension from a GitHub repository:
+
+`gh extension install {{owner}}/{{repository}}`
+
+- List installed extensions:
+
+`gh extension list`
+
+- Upgrade a specific extension:
+
+`gh extension upgrade {{name}}`
+
+- Upgrade all extensions:
+
+`gh extension upgrade --all`