Documentation: Improve README

[amotl]

• Refactor documentation about "standalone" invocation options to separate section. Within the main README, it was too distracting.
• Improve README layout, header, titles, and crosslinking.

[codecov-commenter]

Codecov Report

Merging #653 (c571275) into main (2986165) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #653   +/-   ##
=======================================
  Coverage   46.15%   46.15%           
=======================================
  Files          80       80           
  Lines        3913     3913           
=======================================
  Hits         1806     1806           
  Misses       2107     2107           

Flag	Coverage Δ
unittests	46.15% <ø> (ø)

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