Needed changes for the "validation" section

[aperloff]

• Change the sidebar name of the "Validation" section to "General Advice"
• Change the name "Throughout" to "During"
• Move all overfitting text (i.e. in validation/before/inputs.md) to the validation/throughout/overfitting.md section
• Add equations to the text so it works in dark mode. More technically this means changing the GitHub rendered (i.e. https://render.githubusercontent.com/render/math?math=) LaTeX to inline (mathjax) rendered LaTeX.
• In the validation/before/inputs.md, say we do not typically take into account and "imbalance in nature", but rather have uniform/balanced classes when training. Practically, this means a balance between signal and background, different HT bins, or any other group of classes.
• Add more example code throughout. In most sections, this simply means showing an example of the relevant TensorFlow/PyTorch/ONNX code.
• Add back in the links for validation/before/domains.md, which needs something like, "Last but not the least, a usage of bayesian neural networks has a great advantage..." The problem is that the Bayesian neural network PR hasn't been merged in yet and it's causing the webpage building to fail.

[cmadrid1]

Large portions of this issue was addressed in PR #40. The last part of the issue still needs to be done.