feat: single keyboard repositories

The keyboards repository is large, very large. It is difficult to work with as an external keyboard developer. Ideally, we should be able to reference keyboards in external repositories.

While the build infrastructure in the keyboard repository already supports external reference to other repositories, we have some documentation and process work to complete, and at least 1 blocker -- shared fonts.

* Shared fonts need to be referenced in a .kps file as a remote file reference. Conceptually, any file in the .kps could be a remote file, but for now, the priority is for shared fonts.
* GitHub actions should be leveraged from the ground up for building and publishing keyboards.
* Documentation
* We need an examplar keyboard to be published in its own repository as a starting point
* The keyboard dev documentation needs to be updated to discuss the new model
* Process
* We need to decide what we do if a referenced repository is deleted -- do we simply remove the keyboard from the catalog?
* Or do we cache published versions?
* Do we use GitHub releases?
* How do we establish trust for published packages?
* How does the review process change?
* Do we deprecate adding to the main keyboards repository?

Related:
* https://github.com/keymanapp/keyman/issues/2264
* https://github.com/keymanapp/keyman/issues/10166