Switch all metadata to pyproject.toml

[billbrod]

• Moves everything but the compilation-related info from setup.py to pyproject.toml
• Switch to doing documentation install with optional dependencies instead of a separate conda environment
• Cleans up readme and documentation some
• Changes directory structure to src/ structure
• Updates all links from v1.0.3 to v1.0.4 (which I'll release right after merging this PR)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8955baf) 69.72% compared to head (9aceae5) 69.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   69.72%   69.70%   -0.03%     
==========================================
  Files          19       18       -1     
  Lines        1460     1459       -1     
  Branches      328      328              
==========================================
- Hits         1018     1017       -1     
  Misses        348      348              
  Partials       94       94              

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