Make the Jena javadoc from the release artifacts.

1/ Edit prepare-javadoc
   Set the repo URL number (if from staged release artifacts)
   Set the version numbers.

2/ run "prepare-javadoc"

This downlaods and caches the jars - if you interrupt this step at least
the jarthat was in the process of being downlaoded in case itis
truncated.

3/ Javadoc in "J"

This generates the replacement for "jena-site/content/documentation/javadoc/"
using elephas-index.mdtext and extras-index.mdtext.

main-index.mdtext is an example of the main index.  The script does not
replace "jena-site/content/documentation/javadoc/index.mdtext" - see the
commented out line.