While trying to solve
LUCENE-9495, I thought it might be a good idea to try to simplify the set of variables and properties used during the Ref Guide build.
There are 3 areas to work on:
1. Remove the "barebones-html" build. With Gradle the build is self-contained and gradlew check and gradle precommit could just build the full docs and check them.
2. Remove some properties that only existed for a hypothetical need related to the now-removed PDF.
3. Change remaining properties to be defined directly in build.gradle instead of relying on ant properties functionality.