Add a note that RSA keys are not supported by Cosign

[toddysm]

Description

Some key management systems may default to RSA type keys. We should add a note to https://docs.sigstore.dev/cosign/key-generation that RSA keys are not supported.

[anderssonw]

Agreed, ran into the same issue and made #79.

[Neilblaze]

@toddysm can I take a shot if this is up for grabs?