[FEATURE REQUEST] Link to a source code from OpenImageIO documentation

Too often, documentation is not enough to understand how some API functions are working.
For example, Colour-Science Python library usually provides a link to a source code where possible to check how it works under the hood. (for example https://colour.readthedocs.io/en/master/generated/colour.CHROMATIC_ADAPTATION_METHODS.html )

OpenImageIO can also win from a similar approach. Like in case when user wants to check if there is a additional optimisation in function code, or not all input parameters are documented.

Review how OpenImageIO API documentation is generated and how existing API pages are presented. Compare the requested source links with the current documentation setup, then narrow the work to a testable scope. Done means relevant API documentation pages expose source links where possible and the documentation build verifies them.

Written by the indexing model from the issue text.