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

Nobody has claimed this yet.

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](https://loopback.io/doc/en/lb4/Command-line-interface.html) section, e.g. [DataSource generator](https://loopback.io/doc/en/lb4/DataSource-generator.html)

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](https://www.npmjs.com/package/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.

Compare the CLI Reference pages with the output from `lb4 {command} --help`, starting with the documented DataSource generator and the current help entry points. Define a single Markdown-based source and verify that both the reference guides and terminal help use it; acceptance criteria still need to be established.

Written by the indexing model from the issue text.