Add Schema.org markup

Add the following Schema.org markup to all the pages (including the main page):

* https://schema.org/WebPage

Add the following to all docs page:

* https://schema.org/CreativeWork

https://github.com/dunglas/calavera can be used to extract the relevant metadata from the Git repository. Creating an integration between Calavera and Gastby (a Calavera transformer) would be the best solution.

[Calavera](https://github.com/dunglas/calavera/blob/master/extractor/git.go#L53) is able to generate directly the proper JSON-LD markup (related to #60 and #61).