Refactored SDP Legacy Upgrade content into a separate doc. The SDP
Guide will be remain comprehensive and cover how to upgrade the SDP
itself forwardm from the current version (2020.1) using the new,
p4d-like incremental upgrade mechanism.
The content for manual upgrade procedures needed to get older
SDP installations to 2020.1 is only useful until sites are on
2020.1. This content is extensive, narrowly focused, and of value
only once per installation, and thus the legacy upgrade content
is separated into its own document.
Regenerated work-in-progress HTML files for easier review.