Documentation additions

[ben-nowacki]

Description

Add How-To on using built-in and custom Scalers
Fixed minor doc building and serialization bugs

Related Issues

N/A

How Has This Been Tested?

• Local pytest run
• CI passes

Checklist

• My code follows project style guidelines: nox -s pre-commit
• I have added tests that prove my fix is effective or my feature works: nox -s unit
• I have updated documentation if needed: nox -s docs
• I have linked related issues

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 63.82979% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
modularml/utils/data/comparators.py	0.00%	6 Missing ⚠️
modularml/core/models/torch_base_model.py	50.00%	5 Missing ⚠️
modularml/core/models/__init__.py	33.33%	2 Missing ⚠️
modularml/utils/nn/backend.py	71.42%	2 Missing ⚠️
modularml/core/io/symbol_registry.py	0.00%	1 Missing ⚠️
modularml/core/models/torch_wrapper.py	83.33%	1 Missing ⚠️

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