misc: further streamline the header of our README

[tobiasgrosser]

• Drop 'about' header and inline the content right at the top
• Move the badges before the first header
• Move TOC after the 'about' part

[codecov]

Codecov Report

Base: 90.95% // Head: 90.95% // No change to project coverage 👍

Coverage data is based on head (a3a6631) compared to base (66fd1c8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #319   +/-   ##
=======================================
  Coverage   90.95%   90.95%           
=======================================
  Files          50       50           
  Lines        7064     7064           
  Branches     1167     1167           
=======================================
  Hits         6425     6425           
  Misses        396      396           
  Partials      243      243           

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