feat!: Consolidate Markdown split levels

[benbrandt]

In hindsight, these levels were a bit to granular, which led to some strange splitting behavior. More elements are consolidated into fewer levels to allow for much more natural split points.

Will be a breaking change in the output of the Markdown splitter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (17bc95a) to head (88eae40).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   99.75%   99.72%   -0.03%     
==========================================
  Files           7        7              
  Lines        1603     1432     -171     
==========================================
- Hits         1599     1428     -171     
  Misses          4        4              

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