Server 7.6: Copy new feature descriptions from Release Notes to What's new

We need to fix the inconsistency in how we introduce new features for Server releases. Currently, we introduce new features in two places:

• https://docs.couchbase.com/server/current/introduction/whats-new.html
• https://docs.couchbase.com/server/current/release-notes/relnotes.html

Starting with 7.6 we should always announce new features in the What's New topic (even for maintenance releases.)

Please copy all of the new feature blurbs from the 7.6 release notes topic to the the What's new topic for 7.6. Also, let's add a link from What's new to Release notes, letting people know they can find fixed issues there. And another link from Release notes to What's new, letting readers know they can find new feature descriptions there.