Add documentation for new integrators and SOE methods

[jaabell]

This pull request adds documentation for new analysis components:

New Integrator Documentation:

• ExplicitBathe - Explicit time integration using Bathe's method with built-in numerical damping
  • Second-order accurate with enhanced stability (≈2× larger critical timestep)
  • Two-step integration scheme: t → t+p*Δt → t+Δt
  • Only mass matrix assembly required (no tangent matrix)
  • Includes TCL and Python usage examples
• ExplicitDifferenceStatic - FLAC-style explicit central difference for pseudo-static analysis
  • Leap-frog integration with local non-viscous damping (α = 0.59)
  • Velocity sign memory prevents chatter near zero velocity
  • Suitable for quasi-static and dynamic problems with adaptive damping
  • Includes TCL and Python usage examples

New SOE Documentation:

• Diagonal - Diagonal system of equations solver
• MPIDiagonal - Parallel diagonal solver for distributed computing

Technical Details:

• Both integrators are fully documented with theoretical background
• Mathematical formulations and stability criteria provided
• Comprehensive parameter descriptions and usage notes
• Proper citations to original research papers
• Added to main integrator documentation index

These additions provide users with access to modern explicit integration schemes and efficient diagonal solving methods for large-scale structural analysis.