Introduce content.json / Fix path

This version introduces a content.json file which is generated in the root directory of the website.
It contains the titles and descriptions of each public page and can be consumed by a javascript frontend to e.g. create a dynamic search.

Breaking changes:

• file adapter doesn't strip the prepending "/" in page paths anymore
• the template function isActive expects a "/" for the root path (instead of an empty string)

Full Changelog: v0.3.1...v0.4.0