docs: set up for release documentation. Make changes to publish security.html with CVE announcements referring to the sections in upgrading.html rather than CVE.html. Remove templates.zip as part of html build in Makefile. Also update doc for using CVE.html.

try:
    # Python 3+
    from ssl import SSLError
except (ImportError, AttributeError):
    # Python 2.5-2.7
    from socket import sslerror as SSLError  # noqa: F401