add release notes for 2.0 release

Documentation/release/2.0.md

Documentation/release/dev/catalyst-cxx-11.md

Documentation/release/dev/conduit-0.8.7.md

Documentation/release/dev/conduit-external-support.md

Documentation/release/dev/conduit-impl-state.md

Documentation/release/dev/conduit-internal-targets.md

Documentation/release/dev/conduit-mangle-fix.md

Documentation/release/dev/convert-python.md

Documentation/release/dev/extend_ci.md

Documentation/release/dev/fortran-bindings.md

Documentation/release/dev/node-nelements-method.md

Documentation/release/dev/python-bindings.md