Improve welcome to the project pages

[gvegayon]

Goal

The README.md and home page (sphinx) of the project need to be clearer. Particularly, it should state explicitly what the project is about, talk about pyrenew, and probably provide some examples for new users.

Context

TBA

Required features

• List the project goals and description, linking the wastewater project.
• REAMDME.md should be the same on the Sphinx website and the GitHub repo.
• Show a toy model example (RtInfectionRenewalModel probably).
• Add links to the existing tutorials.

Specifications

• README.qmd file with all the content.
• CI added to the website building, updating the index.html and README.md