Add initial draft for new docs and build with CI

[daurer]

The new docs are build side-by-side with the old documentation. This means the current documentation will remain available on the same URL while we are slowly migrating over to the new documentation pages.

Old documentation page built with CI: https://daurer.github.io/ptypy-new-docs/legacy
New documentation page: https://daurer.github.io/ptypy-new-docs/master

• Add CI for documentation serving both old and new docs
• Configure pydata theme for new sphinx documentation
• Landing page
• API reference
• Parameters
• Add section for User guide with some content from the old docs
• Add section for tutorials with full list of examples from ptycho.github.io/tutorials
• Modify CI to build different versions of the docs (master/dev)
• Modify CI to aslo build docs on release
• Add version switcher to new docs