docs: significant upgrade of documentation

[steven-murray]

A significant upgrade of the documentation.

• Made the README much leaner -- simply pointing to the documentation pages where possible.
• Use the furo theme
• Use sphinx-autoapi instead of autodoc
• Update some outdated installation/developer documentation
• More badges

[codecov]

Codecov Report

❌ Patch coverage is 96.94656% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.72%. Comparing base (2354c9e) to head (88b78fb).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hmf/cosmology/growth_factor.py	94.93%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
- Coverage   97.72%   97.72%   -0.01%     
==========================================
  Files          26       26              
  Lines        2639     2633       -6     
==========================================
- Hits         2579     2573       -6     
  Misses         60       60              

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