Integrate PLDoc server

An additional PLDoc server would autogenerate and serve HTML files for the prolog predicate definitions. This server shall be integrated into the main server.

• The endpoint /doc will serve the main documentation (started in the WIKI).
• The endpoint doc/pldoc will serve the actual predicate documentation.
• Whether documentation is served is configurable using a CLI switch (default is yes)
• The served pages will have some custom look and feel (nice-to-have).
• Footer will display the used SWI-Prolog and WebAPI version.