Documenting the SMC(K) model closes #2382

[hossam26644]

Documenting the smc(k) model.

• Added the smc(k) class description and referred to it in the docs
• Removed and/or replaced mentions of the old smc and smc' models from docs
• Added lines in the old smc and smc' class description referring to them as legacy, and recommended using the smc(k) model
• Added an example of using the smc(k) model to simulate under the smc' approximations
• Added text in ancestry.md mentioning the possibility of using this model to simulate Drosophila, and recommended using the Hudson model whenever possible.

closes #2382

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.22%. Comparing base (819490c) to head (61b1bfd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2444   +/-   ##
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  Coverage   91.22%   91.22%           
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  Files          20       20           
  Lines       11847    11847           
  Branches     2300     2300           
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  Hits        10807    10807           
  Misses        569      569           
  Partials      471      471           

Flag	Coverage Δ
C	91.22% <ø> (ø)
python	98.63% <ø> (ø)

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[jeromekelleher]

Very nice 👍