Add documentation for nanoColor

The API for nanoColor is very similar to full OCIO, so most of the documentation will continue to apply. However, it will be necessary to make a complete pass through the existing documentation to call out any differences.

Start by reviewing the existing documentation alongside the nanoColor API and inventory their differences. Done means the documentation has been checked comprehensively and every relevant nanoColor difference is clearly called out.

Written by the indexing model from the issue text.