update the docs on the docs themselves

add info on the docs plugins/code to docs README,
and update/simplify the documentation re docs in the website and guide;
more desired on this latter but the minimum has been done here I think
fix formatting of download page (new markdown processor)
update javadoc datestamp and simplify it, and remove old stuff from help file
put content in release notes

the template looked foolish (insincere thanks!), plus we are best off updating this as we go along i think
fix download links in guide
add a link on the download page to the latest version snapshot download page
replace references to brooklyncentral repo and examples and other old things