Docs: mention that the Sally inception configuration MUST NOT define a witness in order for the self-OOBI to work.

[kentbull]

Getting the key event log (KEL) and location scheme URLs from the /oobi self-OOBI URL endpoint is only possible if no witness is specified in the inception configuration, which means Sally is being run in direct mode.

Add direct mode documentation indicating that a witness identifier MUST NOT be included in the wits array of the inception configuration for Sally.

[tsterker]

@kentbull would it be fair to have the --direct flag enforce the absence of witnesses in the configuration? Or are there scenarios where this flexibility is more valuable than the guarantee that people don't accidentally rely on witnesses after all?