Use the /signers/registered route to verify if a signer is registered

[jpraynaud]

Issue

There is a new route that lists all the signers registered at a given epoch: /signers/registered/:epoch. We can leverage it to provide a more accurate signer registration confirmation in this guide: https://mithril.network/doc/next/manual/getting-started/run-signer-node#verify-your-signer-is-registered. In order to do so, the current epoch needs to be retrieved first with the /epoch-settings route.

To do

• Adapt the script to verify signer registration
• Update documentation

Note: as the script is shared between current and next version, the guide should be updated in both versions in the same PR