Use a higher resolution PNG version of the icon, matching other
websites related to development.
The previous favicon.ico is from the days of the old Typo3 blender.org
Part of infrastructure/blender-org#57107
This was done originally because we need access to some information that is avalable from `bpy` however, we only need a few pieces of information.
Instead, I have refactored the code to a traditional `conf.py` file that uses template strings that get substituted when the file is copied to the build folder.
The goal here is to make it simpler to adjust the configuration.
This change also moves the config generation from `rna2sphinx` to the main function.
Pull Request: https://projects.blender.org/blender/blender/pulls/119904
