Change homepage layout

[sarinac]

PR for #2098 (update homepage and layout)

The index.rst file divides the content into the following sections:

• Section: h1 Overview (note that I had to section it this way in order to use Sphinx's grid feature and get the CSS flexbox working)
  • section: h2 ArviZ
  • section: h2 Example Gallery
• Section: h1 Key Features - This entire section is a draft. I tossed all the sentences from the existing homepage into this section. (@OriolAbril - feel free to edit this part)
• Section: h1 Support ArviZ
  • div: h3 Contributions
  • div: h3 Citation
  • div: h3 Sponsors
  • div: h3 Donate

LMK what other changes are needed or if the CSS is too messy...(I could try to comment better or reorganize it) 😅

---

:books: Documentation preview :books:: https://arviz--2119.org.readthedocs.build/en/2119/