Closes xpublish-community#160 xpublish-community#161

Closes xpublish-community#164

Closes xpublish-community#160 xpublish-community#161

Closes xpublish-community#164

…into pydata-theme

# Conflicts:
#	docs/requirements.txt
#	docs/source/api.rst

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

Closes xpublish-community#163 xpublish-community#165 xpublish-community#162 xpublish-community#166

# Conflicts:
#	docs/source/api.md
#	docs/source/conf.py
#	docs/source/index.rst