Add docs for using remote signer #149

Per Slack discussion...

@DaveStein asks:

• If you are using the open source SDK and a remote signer... where does OpenSSL come into play?
• Does our SDK require it at all with a remote signer? Is it loaded regardless?
• Do we need to make builds that enforce a remote signer, don't have openSSL attached?
• Do we have samples of using the remote signer in docs anywhere?

We have remote signing support in the open source stuff but it's not documented.

The API is documented in the rust docs. We just don’t have a guide for it.

TO DO: Add a short mention to Signing manifests with link to the Rust API doc and also an FAQ if you want.

API doc: https://docs.rs/c2pa/latest/c2pa/trait.RemoteSigner.html

Maurice: OpenSSL is required as long as the code is verifying. By default the SDK verifies any signature it generates. In a future build check after sign will be optional (it will increase performance for volume signers). When it is not required then we can do build without OpenSSL.