Improve CLI code and descriptions (#28482)

* Close #28444 
* Actually, it doesn't need to use that trick because it looks like it
is not necessary, no user really needs it
* Remove the hidden (legacy) "doctor" subcommand and update documents
* Fix "actions" usage


![image](https://github.com/go-gitea/gitea/assets/2114189/3c2b34a7-4f92-4a6c-96fd-9505e413d4ec)

cmd/actions.go

@@ -15,9 +15,8 @@ import (
 var (
 	// CmdActions represents the available actions sub-commands.
 	CmdActions = &cli.Command{
-		Name:        "actions",
-		Usage:       "",
-		Description: "Commands for managing Gitea Actions",
+		Name:  "actions",
+		Usage: "Manage Gitea Actions",
 		Subcommands: []*cli.Command{
 			subcmdActionsGenRunnerToken,
 		},