Add Backward Incompatibilities and Documentation Enhancements to What's New for Pyramid 2.0

[stevepiercy]

@mmerickel for Pyramid 1.10, we included the sections Backward Incompatibilities and Documentation Enhancements, but they were omitted for 2.0. I'm not sure whether that was deliberate or an oversight.

Should I copy-paste the Backward Incompatibilities section from Pyramid Change History into What's New for Pyramid 2.0?

I can pull out the significant changes from the Issue Tracker labeled "docs" for the 2.0 milestone for Documentation Enhancements, including the following.

• Fix Sphinx 3.0.x build
• update docs to use security policy
• Expand cookiecutters doc to include more references, using the old scaaffolds.rst as an example. The cookiecutter docs now align with Pyramid 2.0.
• Rewrite docs for custom predicates.