Expand tutorials for v1.5.0

[rkingsbury]

Several new tutorials (jupyter notebooks) can be added to help explain new functionality and better orient new users.

1. Update the engines tutorial to incorporate the phreeqc2026 engine
2. add a tutorial on charge balancing, that describes the different strategies available. (This could also be a separate documentation page)
3. Add a tutorial on solid-liquid-gas equilibrium highlighting the new features of equilibrate

[rkingsbury]

@SuixiongTay - charge balancing
@YitongPan1 - solid-liquid equilibrium