Produce Markdown plugin API documentation

Developers can benefit from a comprehensive plugin API documentation.

The currently agreed way to produce this is to:

• run doxygen on the plugin API headers and a specially crafted main page file to produce XML.
• run moxygen (https://github.com/sourcey/moxygen) on the XML file.

So all we need is cmake support for the above. This is what this MDEV is about.

There's a new shell script added to generate the docs (generate_plugin_api_docs.sh). And a cmake target that, when enabled (via the WITH_GENERATED_DOCS) define (off by default) will check the dependencies and call the script to generate the docs as needed.

Note that the script requires at least moxygen 2.1.11.