JBDS update sites need index.html pages to explain how to use Eclipse update

Staging update sites for JBDS such as [0] currently throw 403 errors. We ought to have an index.html page there which borrows heavily from pre-existing instructions [1].

[0] https://devstudio.jboss.com/updates/4.0/staging/techpreview/
[1] https://www.jboss.org/tools/download/installation/update_3_2.html#fromsite

Complete list of sites/folders for which we need landing pages is here [2].

[2] https://devstudio.jboss.com/updates/4.0/staging/