Migrate to book theme

[jacobtomlinson]

This PR migrates the Dask Sphinx Theme to use the Executable Book Theme instead of the Read The Docs theme.

Visually I have updated the navigation to look similar to the previous version of the docs but pages and content should feel modernised and refreshed thanks to the Book Theme's nice design.

Incorporates changes from #53.

Notable improvements:

• Navigation hierarchy is clearer.
  • Top nav is for project level links.
  • Left nav is choosing page within current docs site.
  • Right nav for TOC of the current page.
• Updated fonts and layouts.
• Admonitions look modern and refreshed.
• Scales down to mobile better.
• Scales up to large screens better.

Before

After

More examples

Mobile