blobs: Document the submission process

Add the missing piece of documentation for binary blobs: the submission
process. This follows the external source code process relatively close,
but it is relatively simpler.

The proposal includes a new issue template for requesting inclusion of
blob(s) for a particular module.

Fixes #7516.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
3 files changed