Update README.md

[AlexanderRichert-NOAA]

Summary

This PR reorganizes and shortens the spack-stack README. My main goal is to make it quicker and easier to navigate, especially for new users, and in particular to make the Getting Started documentation easy to find. I've tried to consolidate redundant and related information along with fixing a few typos etc. The main subtraction is that I took out the text about the create env and setup-meta-modules commands since those are on the first page of the documentation (I figure a user at that level of interest in spack-stack is likely to want to read more of the documentation anyway).

Issue(s) addressed

Addresses #746

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[srherbener]

Thanks for this update! I like the trimmed down version for the main README.md page. I think it helps get the main points across more effectively to someone new.

[srherbener]

Thanks for adding in the link to the "creating environments" doc page.

[climbfuji]

Thanks very much, sorry for being so picky.