-----BEGIN PGP SIGNED MESSAGE-----

Hash: SHA1

> How would you digitally sign a Json document and embed the signature in the document?

Embedding a signature into the same file is easy enough.

-----BEGIN PGP SIGNATURE-----

Version: GnuPG v0.9.7 (GNU/Linux)

iEYEARECAAYFAjdYCQoACgkQJ9S6ULt1dqz6IwCfQ7wP6i/i8HhbcOSKF4ELyQB1

oCoAoOuqpRqEzr4kOkQqHRLE/b8/Rw2k =y6kj

-----END PGP SIGNATURE-----

Or use something similar to jwts, you normalize the document, sign the hash, wrap the original document with metadata and include the signature.