TYP/DOC: Improve typign and docs

[bashtage]

Fix small typing issues
Reformat and check notebooks for warnings and errors

[codecov]

Codecov Report

Attention: Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.57%. Comparing base (c5c697e) to head (3181008).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
linearmodels/iv/absorbing.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #626      +/-   ##
==========================================
- Coverage   99.57%   99.57%   -0.01%     
==========================================
  Files         100      100              
  Lines       17267    17281      +14     
  Branches     1443     1445       +2     
==========================================
+ Hits        17194    17207      +13     
- Misses         27       28       +1     
  Partials       46       46              

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