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
```
