Unnest and prune documentation

[nvictus]

• Removed a significant amount of cruft and repetition, and moved a few things around.
• Removed all ..content: directives because the html theme automatically produces a page TOC on the right.
• Flattened the document sections.
• Added a section on "streaming" to the Performance docs. Included examples with polars and duckdb, using both native csv scanners and oxbow.
• Added a link to the repo in the docs navigation (theme option)
• Rewrote the demo notebook