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

Legal Acknowledgement\ 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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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