Use .yml files

The current design has 5 different .bst files that are never actually built. Since they're not intended to be built, we shouldn't think of them as .bst files. They just exist to contain yaml, which other files will @-include, so I think they should be relabelled as .yml files.

Meanwhile, the end-user is suppsed to create actual .bst files in their own project, with @-include statements in each one, pointing back to the files in this directory. But it's not clear how the user is supposed to know which files to create, and what content those files should have.

A good user experience would be for us to provide suitable .bst files, complete with the @-include stements, which the user can then just copy into their own project.