Initial CLI docs (#565)

As title, then we could potentially start ingesting these in the docs repo as they are expanded.

Reviewed-on: https://gitea.com/gitea/tea/pulls/565
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: jolheiser <john.olheiser@gmail.com>
Co-committed-by: jolheiser <john.olheiser@gmail.com>

main.go

@@ -56,6 +56,7 @@ func main() {
 		&cmd.CmdRepoClone,
 
 		&cmd.CmdAdmin,
+		&cmd.CmdDocs,
 	}
 	app.EnableBashCompletion = true
 	err := app.Run(os.Args)