chore: clean up ADR folder and template

[redeboer]

• File structure under docs/adr has been simplified so that:
  • it's easier to reference to files in the sub-folders
  • the resulting URLs in the Sphinx pages become shorter
• The ADR template has been simplified:
  • Merged a few sections
  • Wrapped suggested content in Markdown comments

[codecov]

Codecov Report

Merging #464 (3d4d7a7) into master (ecb6575) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   90.00%   89.94%   -0.07%     
==========================================
  Files          22       22              
  Lines        3333     3333              
  Branches      833      833              
==========================================
- Hits         3000     2998       -2     
- Misses        172      173       +1     
- Partials      161      162       +1     

Flag	Coverage Δ
unittests	89.94% <ø> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
src/expertsystem/amplitude/helicity_decay.py	89.41% <0.00%> (-0.79%)	⬇️