Artifacts not found error in nightly build. Use actions/upload-artifacts@v4 for compatibility with download-artifacts@v4. Also, v3 is being deprecated.
- Streamlined the build of the HTML documentation based on a private docker image.
- Invoke doxygen and Sphinx to create the final HTML pages.
- Employ a cache for the GitHub actions to store the output of the expensive tasks.
- Restructured the GitHub actions builders and introduced a naming scheme.
- Fixed a few typos in the docs.
- Automatically publish the documentation to GitHub pages.