Update documentation

[HealthyPear]

Requirements

• [x] Merge #93 (mainly for because of the Python CI, which is currently failing also in master as it has been introduced by that Pull Request)

Changes

• updated README
• enabled support for pure Doxygen documentation
• default documentation build uses Exhale
• provided conda environment file for developers

I preferred to go for ReadTheDocs instead of GitHub Pages because it gives better support for PR and multi-version builds.

@jimbraun in order to use it, you should just create an account there using your GitHub one, so it can see this repository. I took the freedom to create for you the project there and make myself a maintainer: as soon as you have an account I should be able to see you and add you as a maintainer too.

You can get a peek at the generated Sphinx-based documentation by unpacking this compressed archive.

Hopefully, everything will go smoothly and at some point, something should appear at https://xcdf.readthedocs.io/en/latest/.

[HealthyPear]

I changed the name of the jobs, so you probably need to update your branch protection settings to update the required checks.

[jimbraun]

Thanks much. I agree with the changes and updated the branch protection settings.