[docs] Make it clearer that key generation must be done on the SVS

[eloquence]

https://docs.securedrop.org/en/release-0.11.1/generate_securedrop_application_key.html currently does not explicitly state that the submission keypair must be created on the air-gapped SVS. This may be inferred from the sequence of steps, and from the conclusion ("At this point, you are done with the Secure Viewing Station for now"), but for the sake of avoiding ambiguity, we should make it explicit at the beginning of the document.