Prepare project page update after 3.0.0-alpha1 release

Problem/Motivation

Now that the 3.x series has its first release, the project page needs to be updated to help users choose a version including that one.

The current project page is heavily focused on the stable 8.x-2.x versions, but we need to introduce the appealing 3.x versions without raising undue expectations wrt stability until that branch no longer needs to patch core.

We need to include

• Features of current versions (3.x, 8.x-2.x, 7.x)
• Respective use cases of current versions
• Support availability for current versions
• Notes regarding 8.x-1.x, pre-7.x, and former experiments off d.o. leading up to 3.x

Proposed resolution

• Create the proposed pages as a page in 8.x-2.x with a MR to include it.
• Once we agree on the contents, copy it to the project page
• Close the MR and mark this issue fix.

Remaining tasks