Update the fusion reactions section of the plasma docs to full account for all models and details

[chris-ashe]

The fusion reactions section of the docs needs updated to include and fully explain all new models and variables.

Questions

1. Bosch-Hale presents a parameterised form for the branching ratio for the D-D products, should this be added in since PROCESS will always assume 50-50?
2. The Bosch-Hale method does not include reactions from T-T and 3He-3He, could these be added in?
3. There still seems to be some legacy calculations still used in the main code found in physics_functions.f90. Should these be converted to Python?. Being done by Convert physics functions to Python #3338
4. Some reaction rates may not all be plotted to the output files, this should be checked

[chris-ashe]

This blocked until the Python conversion for some of the physics functions

[mkovari]

This paper (Štancar) seems to say that TT is not negligible. I don't think it's a big deal, though.

I was not aware that the branching ratio for DD is not fixed. What does it depend on?