Intersphinx between IDAES-pse and other docs

[dangunter]

This is two related things

1. Configure intersphinx extension to make it relatively foolproof to link to idaes-ui docs (and in the future others) with Sphinx syntax
2. Normalize look-and-feel between idaes-pse and idaes-ui HTML docs (and in future others?) to make the transition between the two less jarring

for (2) I would propose adopting the "sphinx_book_theme" on the main IDAES pages since it's a bit cleaner and more modern. It does undo some of the custom CSS we had, but I'm not sure that's such a bad thing. See for yourself:

Light mode

Dark mode

[dangunter]

For reference here is a screenshot of the current home page on RTD (the ones in prev. comment are from a local build; on RTD they would have the same advertising junk I'm afraid)