docs: add a new_features.text file and remove 19.3.0 release notes

docs: add a new_features.text file and remove 19.3.0 release notes

The next patch is going to introduce a tool that creates the entire
release html page for us, without any user intervention. As such we
can't be editing it. To that end the script will read the
new_features.txt file to get a list of new features.

This is a flat text file, one entry per line.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Juan A. Suarez <jasuarez@igalia.com>