Add spatial coordinate documentation into the docs folder

[jminor]

Feature Request

Let's add the documentation for the spatial coordinates feature into the OTIO docs.

[jminor]

The docs are here https://opentimelineio.readthedocs.io/en/latest/tutorials/spatial-coordinates.html but there doesn't seem to be a link from the table of contents.

[JeanChristopheMorinPerso]

@jminor someone would need to add it in https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/main/docs/index.rst#tutorials.

[jminor]

Addressed by https://github.com/AcademySoftwareFoundation/OpenTimelineIO/pull/1209