Working on the docs.elgg.org migration

DON'T merge yet, as I'm working on more pages.

this is a work in progress for #6812 

keep in mind the following comment: https://community.elgg.org/discussion/view/1922226/documentation-work

> Also I'd remind reviewers that we shouldn't nitpick at quality when porting docs over from the wiki. Let's just get them transferred and get docs.elgg.org turned down. Then we can review the state of affairs and see what's most needed wrt reorg or quality improvements. That presents docs translators with a small amount of churn but I think based on our development pace it won't actually be that bad (and there aren't many docs translators anyways).

this PR contains the following old docs.elgg.org pages

- [Engine/PageOwnership](http://docs.elgg.org/wiki/Engine/PageOwnership)