docs: rewrite README with experience-first structure

Restructure README to lead with a quick-start example using PSS/E
RAW+DYR files, followed by capabilities, verification, and the
symbolic modeling framework. Replace badge tables and link soup with
four clean badges. List all five routines (PFlow, TDS, EIG, CPF, SE)
as core capabilities rather than separating new features.

- Add readme-figures.ipynb for reproducible figure generation
- Add TGOV1 auto-doc screenshot (replaces IEEEG1 screenshot)
- Add generated TDS and eigenvalue plots
- Remove unused doc-screenshot.png and ieeeg1-screenshot.png

Author: cuihantao