Tutorial on object matrix transformations in docs

Since #5410 suggested we should move valuable stuff from the wiki to the docs, this is a point of some confusion (https://github.com/mrdoob/three.js/issues/1188#issuecomment-3666286 , https://github.com/mrdoob/three.js/wiki/Using-Matrices-&-Object3Ds-in-THREE), I made a short manual entry in the docs explaining the various matrices and how they're calculated/updated---borrowing heavily from @alteredq's and @renegademaster88's descriptions.