[qvtd-dev] Drops and Updates are now Builds

[Edward Willink <ed@xxxxxxxxxxxxx>]

Bottom line: when the partial QVTd distribution as part of SimRel is 
inadequate, full QVTd builds should now be accessed at and below 
https://download.eclipse.org/mmt/qvtd/builds

These builds use the JustJ technology developed by Ed Merks for EMF. 
Many thanks Ed. See https://eclipse.dev/justj/?page=tools and
https://git.eclipse.org/r/plugins/gitiles/mmt/org.eclipse.qvtd/+/refs/heads/master/releng/org.eclipse.qvtd.releng.promote/

TLDR

Traditionally, ZIPs were available from 
https://download.eclipse.org/mmt/qvtd/downloads/drops/ and 
https://archive.eclipse.org/mmt/qvtd/downloads/drops/

Following the advent of P2, repos were supported by 
https://download.eclipse.org/mmt/qvtd/updates/ and 
https://archive.eclipse.org/mmt/qvtd/updates/

The JustJ technology
- folds the zips into the P2 sites making the "drops" tree redundant
- uses "milestone" rather than "stable" for S-builds
- uses "release" rather than "releases" for the release sub-tree
- does not support interim/integration/I-builds
- rebadges the last milestone for a release rather than rebuilding
- enforces nightly/milestone clean-ups; there are no milestones for a 
previous release

These changes make re-use of the existing "updates" tree difficult so 
the new tree is "builds". It contains all recent builds and has links to 
archived builds/drops/updates.

Updating the traditional 
https://www.eclipse.org/mmt/downloads/index.php?project=qvtd query to 
use the new site is a WIP for the EF who are working to discontinue PHP.
Once PHP has gone, the old "drops" and "updates" may be deleted and 
replaced by links to "builds".