Collada user documentation

With the recent addition of Collada animation support !421 (merged), I went and started documenting the said pipeline. A lot of the discussion took place in this thread https://forum.openmw.org/viewtopic.php?f=2&t=3724&start=80 and between me and unelsson on Discord.

There's already solid information included, while some things still need to be researched and documented.

Seeing how documentation is a part of OpenMW's main repository, I presume we're reluctant to add images to the manual? They would greatly enhance usability and appeal of the manual, but increase the size of the repository.