* ports upstream sql changes to docs, closes#4881
* WIP gapfill functions
* WIP gapfill docs
* gapfill function reference
* document how to fill gaps in data with sql, closes#4870
* Apply suggestions from code review
Co-authored-by: Jason Stirnaman <stirnamanj@gmail.com>
* updated phrase of date_bin_gapfill, ported content to dedicated
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Co-authored-by: Jason Stirnaman <stirnamanj@gmail.com>
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