Pattern-file authoring

In this issue, we write the specifications of the structure of so-called 'pattern-files', i.e. files in which an ontological pattern is being specified/described. This top message contains such specifications, and will be edited/updated as needed and agreed. Subsequent messages document the discussions.

1. A pattern file starts as any other DocuSaurus file, i.e. with a DocuSaurus header.
2. The id of the header shall be pattern-{pattern-name}, e.g. pattern-party-action for the Party-Action pattern.
3. The first section shall define the purpose of the pattern, and describe the motivation/relevance for having it.
4. References to images, terminology and other patterns will follow the requirements from issue #10 (closed).

todo: add/change requirements according to the experience we will be gaining when working with this.