add getPackageDocDir function (similar to getPackagePrefix)

[jmairboeck]

An idea I just had:

Would it make sense to add a getPackageDocDir shell function, similar to getPackagePrefix, which returns the package-specific docDir for a subpackage?

$docDir is the only one of the path variables defined by haikuporter that is package specific (i.e. equivalent to $documentationDir/packages/$portName).

I noticed that the opensp recipe (in haikuports/haikuports#11303) now uses $docDir/tools for the tools documentation. Wouldn't a docDir specific to the tools package be more correct?

Do we also need a relative variant to accompany $relativeDocDir? What would be the best syntax for that? A separate function getPackageRelativeDocDir?

Should the result of getPackageDocDir be relative to the package prefix of the subpackage or to the main $prefix (i.e. it should still be listed in packageEntries)? I would tend to the former here, so that it doesn't require packageEntries.