Improving new user documentation

[nefrathenrici]

The current ClimaAtmos new-user documentation needs improvement. We should rewrite the introductory pages with new users in mind.

In particular, we should

• change the landing page to include a more comprehensive description of ClimaAtmos and its current capabilities: microphysics, radiation, thermodynamics, spatial configuration/numerics (?), GPU, calibration, etc.
• change the "Getting Started" page to include an example running a minimal simulation
• add a page with links and brief descriptions of upstream packages to acquaint users with the ecosystem (cloudmicrophysics, thermodynamics, surfacefluxes, atmosphericprofileslibrary)

I tried to keep the scope of this issue small but we can gather other ideas for documentation improvements here. For example, the contributors guide could be more concise and has some rendering errors.

[nefrathenrici]

It looks like the REPL Scripts page is also outdated and should be updated or removed.