Link and embed bundle readme's from Sling Site

As proposed in https://lists.apache.org/thread.html/7cb85c44d3b852fd13acc66413606d1ada1e6585eda2220981ad0c6d@%3Cdev.sling.apache.org%3E we should link

1. from the bundle's readme MD files to the according page below http://sling.apache.org/documentation/bundles.html
2. from the bundles overview page in the Sling Site towards the individual bundle's readmes

The latter should be done automatically by leveraging JBake template functionionality.

Some technical hints on how to implement this:

1. Come up with a new JBake template: bundles
2. This template will use the Github API with its List organization repositories method to figure out all Sling repositories
3. For all repositories containing a readme.md file a link towards this file (on Github) should be placed in the bundle overview page
4. To figure out the relation between individual Sling Site Bundle Documentation pages (e.g. http://sling.apache.org/documentation/bundles/models.html) and the according readme.md, the readme should be optionally extended with a link towards the Sling Site page.
5. TBD: format of the link towards the Sling Site documentation, so that it can be easily extracted with Groovy to put the link to the readme in the right position of the bundle overview page.