Update the plasma profiles section of the docs to include all new models

[chris-ashe]

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

1. Cross relate the sections (impurity, current drive, fusion reaction rate, stellarator)
2. work out how current profiles work
3. Tidy up profiles and plasma_profiles.py
4. Add links to all pages
5. Talk about ions
6. ncore is seemingly ran for non-pedestal plasmas
7. Discuss H-mode
8. ITER profiles and info added from ipdg98