Add stability declarations to documentation

[mdekstrand]

This adds stability declaration syntax to our documentation through a local Sphinx extension, and uses it to declare the stability of several interfaces.

Also modifies the fallback nodes to take primary & fallback inputs.

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.71%. Comparing base (8afcca0) to head (969a0a8).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/types.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
+ Coverage   90.54%   90.71%   +0.17%     
==========================================
  Files         108      108              
  Lines        6556     6540      -16     
==========================================
- Hits         5936     5933       -3     
+ Misses        620      607      -13     

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