Start documentation for internal workings of PyDough

[knassre-bodo]

Includes documentation for the various IRs and conversions between them. The page still has several major TODO sections that need to be filled out, but this PR fills out the following:

• Table of contents and overall structure of the page
• Overview section with flowchart explaining how the pieces fit together
• Entirety of the UnqualifiedNode and unqualified transform sections
• Part of the QDAG node sections, with a diagram showing how an earlier unqualified node looks like as QDAG nodes
• Much of the hybrid tree sections on hybrid conversion & de-correlation with in-depth diagrams
• Part of the relational tree section showing what an earlier example looks like as a relational tree

The markdown page for the IRs includes mermaid diagrams which should render normally on the GitHub UI, or in VSCode if an extension is installed.

[hadia206]

Good job Kian.
Please see my comments below