Convert Sling website to JBake and gitpubsub

I've started experimenting with JBake to generate the Sling website. If that works well we might switch to that + gitpubsub to have a more flexible way to generate the site.

My current experiment is at https://github.com/bdelacretaz/sling-jbake, at this point the site starts looking like the current one and many pages work well.

Update: it's now at https://github.com/apache/sling-site

Internal links will need to be converted, all *.md files need a more complete "front matter" section, currently I have a stub for that, and I think images need to move under the assets folder.

To play with that, generate the site with the bake.sh script (setup shamelessly copied from https://github.com/apache/incubator-tamaya-site)