Update `documentation.yml` to reference docs builder

It seems that recent versions of PHPUnit have started honoring
display_errors. This prompted me to do this more global change, which
means we no longer mean to turn assertions on manually, since that is
done in the development INI file.

Use development file

Let us check that we are running PHPUnit in the right conditions,
otherwise we could get surprises such as no error output.

Check PHP configuration

Unless we specify the working directory, composer will install
dependencies for the project in the root directory.

We want developers to be able to able to generate docs as well. This
means the docs directory should already contain a composer.json for them
to use.

Remove require

I intended to exclude e.g. 11.5.28 but failed to do so because it is
greater than 10.5.49.
Let us instead just check against 10.0.0: if the upgrade fails, it
should be easy to fix it by requiring the correct patch version.

Fix version checking