docs: Add SQL docs for new QUANTILE_CONT and QUANTILE_DISC functions

[alexander-beedie]

• Added Sphinx entries for the new SQL functions.
• Also made a minor clarification to the forward/backward fill definition.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.10%. Comparing base (7472a76) to head (bf2f5a6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19272      +/-   ##
==========================================
- Coverage   80.11%   80.10%   -0.01%     
==========================================
  Files        1526     1526              
  Lines      209338   209338              
  Branches     2418     2418              
==========================================
- Hits       167707   167688      -19     
- Misses      41081    41100      +19     
  Partials      550      550              

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