add documentations

[zshiqiang]

This PR adds documentations including:

• a notebook about the functionality of IndexMapper
• consistent definitions for layers, activation functions, and their formulations (conv2d is excluded since we have Issue #129 about it)
• a tighter lower bound for auxiliary variables in the big-M formulation of ReLU

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By contributing to this software project, I agree my contributions are submitted under the BSD license.
I represent I am authorized to make the contributions and grant the license.
If my employer has rights to intellectual property that includes these contributions,
I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b9600e2) 95.95% compared to head (540b63c) 91.67%.
Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   95.95%   91.67%   -4.28%     
==========================================
  Files          29       29              
  Lines        1656     1730      +74     
  Branches      255      327      +72     
==========================================
- Hits         1589     1586       -3     
- Misses         35       74      +39     
- Partials       32       70      +38     

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