docs: update demo docs for user registration

[conradoplg]

This should merge only after ZcashFoundation/frost-zcash-demo#252 gets merged

[github-actions]

Visit the preview URL for this PR (updated for commit 412dd01):

https://frost-book--pr721-docs-user-registrati-lkb5ck4q.web.app

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[mpguerra]

Looks pretty good, I think the order of actions to be taken by users could be improved a bit, e.g you need to run the participant commands to generate and send the commitments before you can paste the SIGHASH for the message to be signed.
Also, I got an InvalidExternalSignature error when pasting the group signature into the sign command but I think this was probably due to user error.

[conradoplg]

Thanks! I re-tested the flow and it seems the work, the invalid signature error was probably caused by not using the same keys that were used to generate the address (this could happen if you generated them again after you generated the address)

[mpguerra]

Thank you!