Generate API documentation

[osma]

For users and especially developers getting familiar with the Skosmos codebase, it would help a lot to set up browsable API documentation generated using e.g. phpDocumentor.

Similar to this Annif issue

[osma]

Guide for running phpDocumentor: https://docs.phpdoc.org/guides/running-phpdocumentor.html

Basically, we need to add phpDocumentor as a (development) dependency, then try to run it to generate HTML documentation pages. Also adding a configuration file is likely necessary at some point.

When we have this working, the next step would be publishing the generated API documentation somewhere. GitHub Pages? Something else?