AST-919 Publish to ReadTheDocs

• Split 'doxygen' and (sphinx) 'doc' build steps
• Add .readthedocs.yaml with instructions for ReadTheDocs.
• Publish to https://idg.readthedocs.io/en/ast-919-readthedocs-publishing/

@veenboer: Please check if the documentation @ ReadTheDocs is complete.