docs: update doc homepage

[jhamman]

This PR updates Zarr's documentation homepage.

Notes for reviewers:

• This was split off from Doc updates for 3.0 release #2568
• The main motivation here was to provide the core of Zarr's value prop above the fold, as well as links to further reading/documentation.
• I didn't spend time on the styling but I'm open to someone coming along with an opinion on how to improve this.

Marked as a draft until #2590 and #2592 are merged.

[jhamman]

The installation page update was lost in one of the rebases so I pulled in the approved changes here.