Docs revamp

[abkfenris]

Start of a big revamp of the docs.

• Uses Pydata theme
• Enables MyST markdown and converts a lot of the docs to it (rst heading levels and git conflict formatting conflict, which was a fun discovery)
• Re-arranges things along the Diátaxis framework lines, or at least how they are being used in the PyData space
• Adds the vision or 'Why Xpublish' page
• Breaks the tutorial into sections and scaffolds sections for use of plugins

[abkfenris]

RTD Preview

[abkfenris]

We can always continue to tweak the docs later, so going to merge this and work on a release #183