[documentation] Use PyData sphinx theme

• use pydata theme
• make landing page nicer
• add logos of projects/users e.g.
  • https://www.commonwl.org/#cwl-mini-gallery
  • https://cylc.github.io/
• do a quick clean up of docs
  • grammar and spelling
  • fix duplicated sections that appear in the menu (add link to an existing issue I'd almost sure exists)
  • make sure all links are still working, no 404's (links from deliverables, and some used in the cmd line, so important!)

What it looks like, the main page:

A page from a section with pages, and with anchors (links within same page):

The dark mode:

Searching:

On mobile:

Menu on mobile: