Preparation of the web site for the upcoming
v2.1 release:
- there is no longer an install/ subdirectory, but
    there is a install-X.Y.htgt file by version,
    in order to provide history (if someone wants
    to install an old version)
    Note: there is no install-2.0 file.
- relnotes.htgt holds release notes
- upgrade.htgt holds upgrade instructions
    Note: 2.1 upgrade instructions are still to be written
- doc.htgt.* are modified accordingly