Documentation: update discrete_mechanisms/ README for current architecture

[hanzalaareeb]

The README for dpsynth/discrete_mechanisms/ appears to describe an older functional API centered on run_mechanism.
The current codebase uses the higher-level TabularSynthesizer workflow and mechanism utility classes such as AIMConfig, MSTConfig, and MSTMechanism. I am preparing a documentation update to align the README with the current architecture.
Planned changes:

• Replace outdated references to the former functional entry point.
• Explain the relationship between dicrete_mechanism classes and TabularSynthesizer.calibrate().
• Expand the privacy-accounting section to distinguish approximate DP, zCDP, and GDP paths.

I have begun validating the documentation against the implementation and will open a PR once the wording and technical details are verified.
If maintainers prefer a different scope or terminology for this README, I would appreciate guidance before submitting the PR.

[ryan112358]

This would be a welcome contribution, thanks!