Add influence function approximation comparison

In order to get a better overview about the implemented influence function approximation methods and when to choose which, we should add a comparison page to the documentation.

The page should contain information about:

- Computational complexity
- Memory constraints
- Scalability

Look at the existing documentation structure in the repository to understand where comparison pages are placed. Review the implemented influence function approximation methods in the codebase, likely under modules like `influence` or `approximation`. Gather details on computational complexity, memory constraints, and scalability from method docstrings or related papers. Create a new documentation page that summarizes these aspects for each method, ensuring it integrates with the existing docs.

Written by the indexing model from the issue text.