docs/index.rst: Align with latest updates in toplevel README.md

[gmacario]

Closes #239

📑 Description

• [x] Insert ARNEIS logo at the top of the page
• [x] Reorganize sections as in README.md
• [ ] ...

✅ Checks

• [x] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [x] I have updated the documentation as required
• [ ] All the tests have passed

ℹ Additional Information

[gmacario]

FIXME: Why aren't there any new feature builds of the docsite for the last 20 hours???

https://readthedocs.org/projects/arneis/builds/

[gmacario]

NOTE: I purposedly did not update the animation at the top of the page. I will deal with it as part of https://github.com/B-AROL-O/ARNEIS/pull/247