Refined Release Notes and top-level README.md file in preparation
for coming 2023.2 release.
Adjusted Makefile in doc directory to also generate top-level
README.html from top-level README.md file so that the HTML file is
reliably updated in the SDP release process.
Updated :revnumber: and :revdate: docs in AsciiDoc files to
indicate that the are still current.
Avoiding regen of ReleaseNotes.pdf binary file since that will
need at least one more update before shipping SDP 2023.2.