Add some auto config via hooks examples (e.g. setting arbitrary `config.php` values via `occ config:system:set`

[joshtrichards]

It's not necessary to add every possible config scenario as a dedicated environment variable in the image. I think we can expand the docs for #1964 a bit to give people a clearer idea of how they might use hooks for auto configuration.

Should cut down on new environment variable requests (and without resorting to NC_* usage which is incomplete and undocumented, upstream, at best).