Write examples/extra folder as Jupyter notebooks

[quantumsteve]

This updates comments from # to ## in the examples/extra folder so that they can be written as Jupyter notebooks.

[kbarros]

Big picture comment -- files in examples/extra are not really intended as tutorials for end users. Perhaps they should be better hidden, rather than exposed via notebooks?

• An exception is the parallel tempering example -- many users will want this, so it is is a good candidate to be cleaned up and included as a full-blown tutorial in the main docs. Probably this work should happen in conjunction with a review/refactor of the MC functionality, so I'm not requesting a contributed PR yet.
• The other Advanced_MC examples involve Wang Landau algorithm which is more niche, and less accessible to typical Sunny users. Probably these examples should stay hidden until we have a strong use case that would motivate writing user-friendly tutorials.
• The files in KPM were really only intended for us, as Sunny developers. Maybe they should just be removed so as not to confuse Sunny users who are browsing around?
• The Plotting directory was intended to hold helper functions that can help simplify the main tutorials. See, e.g., the how plots are made at the bottom of the CP2 skyrmion tutorial
  In particular, I don't think it makes sense to expose these files as notebooks.
• heisenberg_animation.jl is sort of an odd-duck -- It's a nice illustration of creating interactive 3D graphics, but the animations don't easily translate to a web-browsable tutorial in the main Sunny docs.

[kbarros]

Since examples/extra are not intended for notebook generation, I'm closing this PR. But in the future, we should revisit the question of making some of these examples more appropriate as full tutorials.