Drop smallpt example notebook

[mcbarton]

In this comment #415 (comment) and on Discord @anutosh491 mentioned that the smallpt example isn't really adding much value to the repo, and is likely to be dropped before the next release. This PR removes the smallpt notebook from the repo, and any reference to it in the documentation.

[anutosh491]

Drop all the SDL changes from Cmake too I guess!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.94%. Comparing base (19c639b) to head (7042815).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #422   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files          21       21           
  Lines         853      853           
  Branches       87       87           
=======================================
  Hits          699      699           
  Misses        154      154           

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[mcbarton]

Drop all the SDL changes from Cmake too I guess!

I did wonder whether to do this. I have done this now.

[anutosh491]

Thanks, I can add a better ray tracer example soon.