Add features tied to Royal Gold funding goal

[squidfunk]

This PR includes all features related to the Royal Gold funding goal which was just reached. Since I'm going to be mostly offline until mid-August, and I want to make sure that existing workflows do not break, 8.4 is going to be a pre-release until I'm back and things can be considered stable. I've tested all features and I'm reasonably convinced that the merge is complete and everything works as expected, but I'm not perfect, so there might still be bugs hiding somewhere 🐞

If you want to upgrade to 8.4, you can do so with:

pip install mkdocs-material==8.4rc1

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