Add `Packaging` subsection

[vincele]

When I started building packages for void, I searched for documentation. I initially did not find it, despite having skimmed the chapter containing it. I then proposed to add something to the doc in #754.

After revisiting the issue and searching more thoroughly (someone gave an actual pointer to the right location) I think this can be made a bit more visible by creating a subsection specifically for it.

I don't think the (small) additionnal space is lost, but if it can help myopic readers, why not.

[vincele]

And then there's this other related PR: #639.