[shimwell]

Adds FESTIM to both installation examples.

• The pip path installs FESTIM and imports it in the heat-equation script exercised by CI.
• The conda path includes FESTIM for parity, although that combined environment is already documented as unresolved.

The pip workflow will show whether FESTIM can import with the published FEniCSx wheels.

[shimwell]

@RemDelaporteMathurin @jhdark with this merged the pip environment has everything needed to add in a minimal festim example which takes the heating output in vtkhdf format by 2-solve-heat-equation.py and the mesh (also in vtkhdf format) and solves tritium. What do you think would that be worth adding?