* Moved navbar elements in all html documentation files to partial

This consolidates the common menu elements in all documentation files to a partial.

* Moved test-exclamation mark

* Add template for document generation with 'jsdoc'.

In essence, it defines the subdirectory `docs` as a `jsdoc` template. Benefits:

- allows the usage of partials, in order to DRY common parts of the html files.
- makes available the jsdoc-comments, for addition into the documentation.
- enables extraction of data from the source code. For example, the list of edge endpoints `['arrow', 'bar', 'circle']` can now be extracted from the source and inserted into the documentation on generation.

In this initial version, the only file that has been changed is `docs/data/dataset.html`.
In here, partials have been added to illustrate how common page elements can be DRY'd.

The template has been set up in such a way, that resource files will be copied and that html files can pass through unchanged if no special template tags (`<?js...?>`) are used. This allows for a gradual transition of the html files to templates.

**Usage:**  `gulp docs`

- The result files are placed in subdirectory `gen/docs/`.

**NOTE:** The release procedure will have to be adjusted by adding this gulp command.
The docs-files will then have to be taken from `gen/docs`.

* Edits to docs/README

* Adjusted layout of README.md

* Further edits to README.md

* Removed pre tags again in  README.md - don't work in code block

* Linted the gulpfile

* Added proof of concept for docs generation from source