Seperate out phpdoc to docker based action

[ajparsons]

There's something wrong with the current phpdoc process - I think it's a dependency versioning issue that will go away when we upgrade the PHP version to 8 at some point and can use the newer version.

BUT - we can also just move all of this into github actions. This creates a new action that uses the docker version of phpdoc and uploads directly to github pages without pushing back to a branch.

(In principle if we stick with this we could remove the dependencies from the repo, but also it seems fine)

[ajparsons]

I've removed the dependency and all the tests seem fine - so will merge that as well.

[dracos]

You could also have removed build-docs.sh :)