Convert code-blocks to literalincludes in OpenEXRCoreAPI.rst

The example code in [website/OpenEXRCoreAPI.rst](https://github.com/AcademySoftwareFoundation/openexr/blob/main/website/ReadingAndWritingImageFiles.rst) should be moved to individual files in [website/src](https://github.com/AcademySoftwareFoundation/openexr/tree/main/website/src) and included in the documentation files via `.. literalinclude::`, and also included in [website/src/all.cpp](https://github.com/AcademySoftwareFoundation/openexr/blob/main/website/src/all.cpp) so that it get built by CMake for testing and validation. See the other uses of literalinclude as examples.

This task requires some basic familiarity with [reStructuredText](https://docutils.sourceforge.io/rst.html) and [sphinx](https://www.sphinx-doc.org/en/master) but the concepts are easy to pick up from other examples. See [Building the Website](https://openexr.com/en/latest/install.html#building-the-website) for how to test the formatting.

Compare other literalinclude uses in the documentation, then inspect website/OpenEXRCoreAPI.rst, the individual files under website/src, and website/src/all.cpp. Move the example code into separate source files, reference them with literalinclude, and add them to all.cpp so CMake builds them. Follow Building the Website to verify formatting and validation.

Written by the indexing model from the issue text.