Improve "gitea doctor" sub-command and fix "help" commands (#26072)

Replace #21790

And close #25965 by the way (it needs a separate fix for 1.20)

Major changes:

1. Move "gitea convert" to "gitea doctor conver". The old "gitea doctor"
still works as a hidden sub-command (to avoid breaking)
2. Do not write "doctor.log" by default, it's not useful in most cases
and causes bugs like 25965
3. Improve documents
4. Fix the "help" commands. Before, the "./gitea doctor" can't show the
sub-command help correctly (regression of the last cli/v2 refactoring)

After this PR:

```
./gitea help # show all sub-commands for the app
./gitea doctor # show the sub-commands for the "doctor"
./gitea doctor help # show the sub-commands for the "doctor", as above
```

1 files changed, 0 insertions, 56 deletions

diff --git a/cmd/convert.go b/cmd/convert.go
deleted file mode 100644
index 37a260cbd8..0000000000
--- a/cmd/convert.go
+++ /dev/null
@@ -1,56 +0,0 @@
-// Copyright 2019 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package cmd
-
-import (
-	"fmt"
-
-	"code.gitea.io/gitea/models/db"
-	"code.gitea.io/gitea/modules/log"
-	"code.gitea.io/gitea/modules/setting"
-
-	"github.com/urfave/cli/v2"
-)
-
-// CmdConvert represents the available convert sub-command.
-var CmdConvert = &cli.Command{
-	Name:        "convert",
-	Usage:       "Convert the database",
-	Description: "A command to convert an existing MySQL database from utf8 to utf8mb4 or MSSQL database from varchar to nvarchar",
-	Action:      runConvert,
-}
-
-func runConvert(ctx *cli.Context) error {
-	stdCtx, cancel := installSignals()
-	defer cancel()
-
-	if err := initDB(stdCtx); err != nil {
-		return err
-	}
-
-	log.Info("AppPath: %s", setting.AppPath)
-	log.Info("AppWorkPath: %s", setting.AppWorkPath)
-	log.Info("Custom path: %s", setting.CustomPath)
-	log.Info("Log path: %s", setting.Log.RootPath)
-	log.Info("Configuration file: %s", setting.CustomConf)
-
-	switch {
-	case setting.Database.Type.IsMySQL():
-		if err := db.ConvertUtf8ToUtf8mb4(); err != nil {
-			log.Fatal("Failed to convert database from utf8 to utf8mb4: %v", err)
-			return err
-		}
-		fmt.Println("Converted successfully, please confirm your database's character set is now utf8mb4")
-	case setting.Database.Type.IsMSSQL():
-		if err := db.ConvertVarcharToNVarchar(); err != nil {
-			log.Fatal("Failed to convert database from varchar to nvarchar: %v", err)
-			return err
-		}
-		fmt.Println("Converted successfully, please confirm your database's all columns character is NVARCHAR now")
-	default:
-		fmt.Println("This command can only be used with a MySQL or MSSQL database")
-	}
-
-	return nil
-}