Documentation updates

[jdebacker]

This PR:

• Adds a script to create images for the docs
• Updates the docs to:
  • Update the tax functions chapter to references just the linear tax functions used in OG-ZAF
  • Updates figures in the ZAF demographics chapter
  • Remove the chapter on the bequest transmission process
  • Updates the description and images for the earnings process chapter
  • Puts a "to be completed" not in the matching labor, income, and wealth moments chapter

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.07%. Comparing base (602d724) to head (621ebb8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   56.46%   55.07%   -1.39%     
==========================================
  Files          12       12              
  Lines         356      345      -11     
==========================================
- Hits          201      190      -11     
  Misses        155      155              

Flag	Coverage Δ
unittests	55.07% <ø> (-1.39%)	⬇️

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[rickecon]

@jdebacker. Looks great. Will merge as soon as all tests pass.