provide space for new 2.0 changes not part of upgrade path

provide space for new 2.0 changes not part of upgrade path

As we are going to be adding new improvements such as
variant, cython, psycopg3, more RETURNING support,
fast execution helpers, and probably a lot more, the
2.0 migration doc needs to also still have the "what's new in 2.0?"
thing going on as well.   Organize the document so it has space
to introduce these sections, as well as the existing
1.4->2.0 migration sections.

If the "What's New" document gets large, we will break up this
page into two separate pages, it's already quite large, so
the proposal would be the exising migration doc becomes
a separate special migration document.

Change-Id: I62496b30229806f4a82d1f92b3f4eda53e64df57