Smart factors docs

[dellaert]

• added SmartFactors.md, which gives detailed math and implementation details for smart factors
• RegularHessianFactor: Format, document, optimize slightly - Gemini 2.5

[Copilot]

Pull Request Overview

This PR adds detailed documentation for smart factors including mathematical derivations and implementation details as well as several formatting, documentation, and optimization improvements for the RegularHessianFactor class. Key changes include:

• Adding the SmartFactors.md file with comprehensive math details and usage examples.
• Updating the Colab URL in the EKF_SLAM notebook to reflect the correct repository structure.
• Enhancing documentation, error messaging, and optimizations in RegularHessianFactor.h.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
python/gtsam/examples/EKF_SLAM.ipynb	Updated the Colab badge URL to point to the correct file location.
gtsam/slam/doc/SmartFactors.md	New documentation file detailing the mathematics and implementation of smart factors.
gtsam/linear/RegularHessianFactor.h	Refactored and documented methods with performance optimizations and enhanced error messages.

Comments suppressed due to low confidence (1)

python/gtsam/examples/EKF_SLAM.ipynb:33

• Ensure that the updated URL accurately reflects the repository structure and intended location.

<a href="https://colab.research.google.com/github/borglab/gtsam/blob/develop/python/gtsam/examples/EKF_SLAM.ipynb" target="_parent"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"/></a>

[dellaert]

Merging without review as functionality-neutral