Make pmm-admin --help more user-friendly

What should be done

1. Move detailed help for the "pmm-admin add" command under "pmm-admin add --help".

2. Hide additional information in "pmm-admin --help" command, in the "add external-serverless" section (see screenshot), but keep it in "pmm-admin add external-serverless --help" 

3. Sort commands in "pmm-admin --help" and the subcommands alphabetically keeping "help" command first

Why

• Currently all subcommands are listed which makes it confusing when trying to understand which commands are available on the top level.

• The help is confusing and a lot of text when user is not asked for this 

Benefit

Easier to understand and cleaner help documentation

Details

Current state

Suggested state