docs(generate): Improve jumping off points

[epage]

This provides a skeleton README for clap_generate, including a
one-line summary and a link to the docs to guide people to the examples
there.

In doing so, I tried to clarify and be consistent in what role
this crate plays. While it is very general, being too general in the
description can lead people to not understand where they could use it.

Fixes #1711

[pksunkara]

What do you think about some small blurb about getting started in src/lib.rs as doc for the library at the beginning?

[epage]

What do you think about some small blurb about getting started in src/lib.rs as doc for the library at the beginning?

Directed people to some next steps but I've kept it limited to not grow the scope of this PR. In writing this, I've noticed some polish that clap_generate could use, like

• impl ArgEnum for Shell, which will require including the derive traits without the derive feature which I think we should do
• ArgEnum::generator(self) -> &dyn Generator

With those in place, we can tell a better story for how to use clap_generate (even better is to provide clap::Args and clap::Subcommand impls, even if hand-generated, to get people up and running faster, whether using derives or builder, see #810).

[pksunkara]

even better is to provide clap::Args and clap::Subcommand impls, even if hand-generated, to get people up and running faster, whether using derives or builder, see #810

Yup, that was my plan to fix that issue.