feat(navigated-docs): add template for docs

Grouped documentation may now be created using the
navigated-docs template.

A file including the navigated-docs template MUST be empty
(except for meta data) and MUST declare a part variable in
its meta data.

Example:

---

layout: 'navigated-docs'
part: 'api-references/rest'

---

The part variable represents the path under which all documents
that should be rendered are to be found.

Each rendered document must include two fields in its meta data:
- title: the title of the document
- category: the category it is displayed under

Example:

---

title: 'Delete Authorization'
category: 'Authorization'

---

Both, categories and files are ordered according to their names,
that is a category named "00-asdf" is rendered before the category
"b" and a document entitled "00-introduction" is rendered before the
document "other-document".

Related to CAM-1120