Incorporate the old physics section of the documentation

[isaacgsmith]

Problem description

We would like to incorporate the old physics section of the documentation into the newer physics section.

Expected Output

Currently, the old physics section contains two files of interest: docs/physics/old_physics/montecarlo_old.rst and docs/physics/old_physics/old_plasma.rst. The following changes need to occur:

• Make sure all important information in docs/physics/old_physics/montecarlo_old.rst is included within docs/physics/montecarlo/.
• Make sure all important information in docs/physics/old_physics/old_plasma.rst is included within docs/physics/plasma/.
• Delete docs/physics/old_physics/ and remove it from the toctree in docs/index.rst.
• Make sure any hyperlinks referencing a document in docs/physics/old_physics/ is changed to reference the appropriate still existing document.

[jaladh-singhal]

@marxwillia now that @smithis7 has laid out all the steps, it can be done quickly.