[WIP] [docs] filter by tags

[maxpumperla]

A filter-by-tag system that can be used across all our galleries. The "trick" is to attach a new class name to the button of a panel item (no other way known to me to modify the meta tags of sphinx panels) and then toggle based on tags found in a class to tag mapping JSON that can be fetched from static files at runtime. To add other galleries, we can simply extend the JSON mapping, if needed.