Make the home page pretty

[stevepiercy]

Links of affected pages in Plone Documentation, if any.

https://6.docs.plone.org/

Description

The home page is not great. For inspiration, see:

• https://numpy.org/doc/stable/
• https://pydata-sphinx-theme.readthedocs.io/en/stable/

We can use Sphinx Design to have a card-based layout.

We currently have no User Guide. See #1611 for the plan.

We have solid "Contributing guide" and "Get started", and the beginning of both "Developer guide" and "Conceptual guides".

We need a new page for "Reference guide" that includes links to any reference material, including plone.api and Plone REST API.

[Manas-Kenge]

Hey @boss6825 , I raised a pr for this issue as it was inactive for more than a month. I hope you don't mind