Consolidate DataFrame Docs: Merge HTML Rendering Section as Subpage (#1161)

* docs: unify dataframe documentation (#2)

* docs: update links in basics and rendering documentation

* docs: add API reference section to main index

* docs: add license information to rendering documentation

* Move API Reference under User Guide > Dataframe

* Merge data from dataframe api reference page into main dataframe page

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Co-authored-by: Tim Saucer <timsaucer@gmail.com>

Author: kosiew