Fix documentation generation

[peternewman]

This doesn't install due to security issues with the version of Twig it uses.

This looked the most compatible, but still had issues:
https://github.com/evert/phpdoc-md/compare/master...LulububuSoftwareGmbH:phpdoc-md:master?w=1

This isn't released as a package, and still needs an old twig, but has an option to do it natively in PHP which might solve the issue a different way:
https://github.com/evert/phpdoc-md/compare/master...jclaveau:phpdoc-md:master?w=1

Originally posted by @peternewman in #74 (comment)

Hello! Thanks very much for contribution! I guess we can get rid of the evert/phpdoc-md. For now we will rely on the docstrings. I am even open to recreating the docs from scratch. To be honest - I was quite young and stupid when I was proposing this kind of docs all that years ago.

Originally posted by @Sibyx in #74 (comment)

[peternewman]

I guess we can get rid of the evert/phpdoc-md. For now we will rely on the docstrings. I am even open to recreating the docs from scratch. To be honest - I was quite young and stupid when I was proposing this kind of docs all that years ago.

Originally posted by @Sibyx in #74 (comment)

I assume that's what's generating https://sibyx.github.io/phpGPX/ isn't it? There were also some similar PHP to Markdown tools available, so one of those may work, or it's also possible to skip the security warnings, but I didn't think I should bake either of those into the Actions running.

Presumably something needs to be done in some way, so new docs are kept up to date. Otherwise does Doxygen support PHP?