Fusion cross section documentation

[roelof-groenewald]

          I noticed our parameter https://warpx.readthedocs.io/en/latest/usage/parameters.html and also theory section are a bit sparse on the models we use for fusion cross-sections.

Feel free to add a follow-up PR that links your figures @PhysicsDan into the user-facing manual. (Please use external links to figures in the GitHub PR description instead of adding them in Git, which keeps git clone times of WarpX small.)

Maybe as a subsection of https://warpx.readthedocs.io/en/latest/theory/collisions.html or as a new section in it? :)

Originally posted by @ax3l in #4377 (comment)