feat(keymap): create master header for dt-bindings imports

[huber-th]

Resolves #582

Create a single header file to include in keymaps to allow users to use any behavior without having to remember to add additional includes.

Open questions:

• Did I miss spots in the docs which should be adjusted?
• Should the include sections in the behavior docs be completely removed instead of having the info box?
• Should all existing keymap files be changed to use this new #include <dt-bindings/zmk/all.h>?