Small fixes to the README

[niksirbi]

• Somewhat simplified installation instructions
• Fixed links to NPG's logo and social preview images

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.95%. Comparing base (c3e7e57) to head (e924629).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   54.95%   54.95%           
=======================================
  Files          42       42           
  Lines        3683     3683           
=======================================
  Hits         2024     2024           
  Misses       1659     1659           

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[niksirbi]

The Sphinx build is breaking because fixing the image links for the README broke the paths for the website's homepage.

That's because the current website simply includes the README.md file as is, which is doomed to fail. Image links, admonitions and many other elements will fail to properly render with that approach.

Either we have to invest significant effort in building a proper docs website for NeuralPlayground (see #40), or we have to remove the current website elements (and related CI jobs).

[niksirbi]

In a call with @rodrigcd today we discussed that we'll probably get rid of the current prototype Sphinx website, so we can ignore the failing Build Sphinx Docs action here.

That actions workflow should be removed together with the docs folder.

[niksirbi]

I've rebased this PR to main, following the merge of #137.