CLI docs: unify reference guides and `lb4 --help`

At the moment, we have two different content describing CLI commands:

• lb4 {command} --help prints content generated by Yeoman based on generator arguments and options
• Hand-written documentation pages in CLI Reference section, e.g. DataSource generator

It would be great to unify these two kinds of resources into a single source. (This idea was originally mentioned by @raymondfeng.)

For example, we can maintain CLI help in Markdown format and use a module like marked-terminal to allow lb4 --help to render the documentation to terminal. (Tangentially related: Get rid of Yeoman #844)

This story is a part of #5113 Documentation improvements 2020Q3

Acceptance criteria

TBD - will be filled by the team.

Confronta le pagine di CLI Reference con l’output di lb4 {command} --help, iniziando dal generatore DataSource documentato e dagli attuali punti di accesso all’help. Definisci un’unica fonte basata su Markdown e verifica che la utilizzino sia le guide di riferimento sia l’help del terminale; i criteri di accettazione devono ancora essere stabiliti.