Update plasma documentation to show and demonstrate all models and variables

[chris-ashe]

The documentation for the plasma is currently only one page and does not describe the operation of all the models and how to properly set up the variables

• surfa() in plasma_geometry.py seems to be defined but never called
• ieped == 2 has PLASMOD relation and links
• There is no reference for how the core plasma density function is derived
• bokeh > 3.0 may be required
• current profile needs to be covered in physics.f90 not in profiles
• say about Greenwald limit in profiles

[chris-ashe]

Due to this issue not being attended to in a while the plasma docs have now just been split into their separate sections. This will allow separate issues to be made to update each part and ensure that new updates are able to be rolled out quickly.
#3195