Improve quickstart - Introduction

[mariaschuld]

This is the first of a series of PRs to make some temporary fixes to improve the outdated quickstarts, before they will face a more concerted upgrade.

Part I - Introduction

• Rename to "in a nutshell"
• Update text to be more general
• Remove overview of other sections. They should have self-explanatory titles, instead of this hidden, hard-to-maintain section.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2530 (c568696) into master (e326486) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2530   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files         243      243           
  Lines       19405    19405           
=======================================
  Hits        19314    19314           
  Misses         91       91           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e326486...c568696. Read the comment docs.

[josh146]

Thanks @mariaschuld! This is a nice, clean update that is much better than the previous version. I only have minor comments regarding how we first position PL in the first few paragraphs

[josh146]

Looks great!