docs/random: update moving-plugins document and move it to gst-docs
@thaytan pointed out that we have documentation for how to move plugins in #474 (comment 363387) (I wasn't aware of that). It is a bit outdated in technical terms (mentions CVS, mentions autotools), but the fundamental requirements haven't changed.
- Update the document in light of our current development tools
- Maybe refactor it to make a distinction between the preliminary work that anyone can do to help making sure an element meets certain standards from the final work needed to actually move the plugin
- Port the document to commonmark and move it to our main documentation, I suggest under https://gstreamer.freedesktop.org/documentation/additional/index.html