docs: improve pure pursuit documentation

[Messi11111]

Summary

This PR improves the documentation for Pure Pursuit path tracking by adding:

• Algorithm overview
• Geometric intuition
• Steering law derivation
• Implementation details linking theory to code

Motivation

The previous documentation provided only a brief description and animation, which made it difficult for beginners to understand the mathematical intuition behind the controller.

This update aims to make the Pure Pursuit implementation easier to understand for readers learning the algorithm.

[AtsushiSakai]

@Messi11111 Thank you for your PR. Could you please indent the alpha equation under .. math:: and check the generated HTML? Please also use atan2(y_t - y_r, x_t - x_r) to match the code.

Please check How to contribute.