docs: port older "Bridgetown Cards" to new Cookbook docs

[jaredcwhite]

We used to have a separate "Bridgetown Cards" website with some recipes for solving various interesting tasks in Bridgetown, then that got taken down and we had a section of a community Lemmy site for such a thing, then that went off line and now we have a Cookbook section of the docs proper.

We should fish around in the old Cards repo for anything still relevant and add those recipes to the Cookbook. Then we can remove the repo.