Update physics-based prognostic models notebook

[lymichelle21]

• Add table of contents
• Fix formatting for consistency

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[kjjarvis]

Two very small edits but this looks great and everything runs successfully.

[teubert]

Great work. Added a few minor comments.

[teubert]

Organization thought. Maybe we should put all these features into a section for ease of navigation. So the sections would be:

• Linear Models
• State Transition Models
• Advanced Features - with everything there as subsections
• Simplified Battery Model Example
• Conclusion

What do you think?

[lymichelle21]

Having an advanced features section makes sense since the subsections like derived parameters, matrix data access, state limits, custom events, and serialization all focus on optional features for the models.

I'm assuming direct models should be its own section instead of a subsection?