Migrate PyRAMSES Sphinx documentation to main website

Complete migration of all content from pyramses.sps-lab.org:

pyramses/installation.mdx: MKL dependencies, Gnuplot setup, verification
pyramses/overview.md: Package attributes, main classes, platform support
pyramses/api-reference.md: Complete API for cfg, sim, extractor classes
  - All methods with parameters, return types, and examples
  - Runtime observables (BV, MS, BPE/BQE, ON, TO, RT)
  - Extractor with all sync attributes (P,Q,S,A,FV,FC,T,ET,FW,DD,QD,QW,SC)
pyramses/examples.md: All 9 examples from Sphinx docs
  - Basic simulation, pause/continue, state queries
  - Runtime disturbances, result plotting, parameter sweep
  - Eigenanalysis workflow, Nordic generator trip, 5-bus exciter change

solver-settings.md: Added $OBS_BUFFER_SIZE, $SPARSE_SOLVER
test-systems/5bus.md: New 5-bus test system page
astro.config.mjs: Added Test Systems sidebar section

Author: apetros