Fix broken equation rendering in user guide

[schroedtert]

Summary

Fix incorrectly rendered equations in the user guide and add a pre-commit RST lint check for docs/. Equations using :math: and .. math:: directives in Python docstrings are not displaying correctly in the built documentation.

Background / Context

PedPy's method documentation relies heavily on equations to describe the implemented algorithms. Broken rendering degrades usability for the target audience. The existing pre-commit config covers Python linting and notebook stripping but nothing validates RST/math markup, so regressions go undetected until a docs build.

Technical Details

• Equations live in docstrings throughout pedpy/methods/ — e.g. acceleration_calculator.py, speed_calculator.py, density_calculator.py, profile_calculator.py
• RST source files are under docs/source/
• Pre-commit config: .pre-commit-config.yaml
• Candidate hooks: sphinx-lint or rstcheck — both support pre-commit and catch malformed math directives

Acceptance Criteria

• All equations render correctly in the ReadTheDocs build
• A pre-commit RST lint hook is added to .pre-commit-config.yaml covering docs/source/**/*.rst
• No math-related Sphinx warnings in the ReadTheDocs build log