[docs] Switch to 'Furo' theme for core docs

[Web3NL]

Closes #1573

Problem

Current 'Alabaster' theme does not support mobile and dark mode

Proposed solution

Use 'Furo' theme. No particular technical reason for this specific theme, apart from esthetics

Todo

• Since I'm not very familiar with Sphinx, I might have missed something. Needs checking.
• Testing required

Building

HTML builds with same command on WSL2 setup: make html

Preview

https://eeqt3-riaaa-aaaal-qbsva-cai.ic0.app/

Screenshots

Desktop light

Desktop dark

Mobile