chore: sharing schemas between repositories

We have a fairly extensive set of schemas, now at:

* https://github.com/keymanapp/api.keyman.com/tree/master/schemas
* https://github.com/keymanapp/keyman/tree/master/common/schemas
* and some specific schemas scattered elsewhere

It's tricky keeping these synchronized. If you look at these two links, you'll see that they are not consistent.

1. We should have a single common source of schemas
2. This needs to be synchronized with each downstream repo that uses it
3. The documentation should be a separate file associated with each schema (currently on api.keyman.com, it's a single README.md)
4. Schema versions need to be very clear

git submodules feel like overkill. Other sharing solutions feel like bandaids too. Manual copy may still be the best approach, but if so, we need to cleanup the structure and make it consistent and easy to trace (perhaps a script?)

* The synchronization may be best solved by using the way we copy files over to help.keyman.com during builds.
* keymanapp/keyman should be the master version, not api.keyman.com