Put list of available modules into PHP track documentation

[mk-mxp]

There is a list of PHP modules in php-test-runner, that we want to be in the student facing track documentation on the website.

Choose a place in the track docs and insert the list there.

[hiSandog]

The module list should ideally be generated from or checked against the exact php-test-runner image manifest, otherwise student docs will drift when extensions change. The page can include runner/PHP version, enabled modules, and a link to the source image, with a note that local PHP may differ. A CI comparison between documented module names and php -m from the current runner would keep the list trustworthy.

[mk-mxp]

For the track docs (not the READMEs): link the modules to the place in the PHP documentation that lists all their functions.