doc: add Make target to run phpDocumentor with Docker

This target provides a convenient way for running phpDocumentor without
cluttering the system's configuration with PHP extensions, nor the
Composer dependencies.

See:
- https://hub.docker.com/r/phpdoc/phpdoc/dockerfile
- https://github.com/phpDocumentor/phpDocumentor2#via-docker

An alternative is to download the PHAR and run it locally:
- https://docs.phpdoc.org/getting-started/installing.html#phar

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

Makefile

@@ -151,6 +151,10 @@ doxygen: clean
 	@rm -rf doxygen
 	@doxygen Doxyfile
 
+### generate phpDocumentor documentation
+phpdoc: clean
+	@docker run --rm -v $(PWD):/data -u `id -u`:`id -g` phpdoc/phpdoc
+
 ### generate HTML documentation from Markdown pages with MkDocs
 htmldoc:
 	python3 -m venv venv/