Extend Ledger docs

[CtrlAltDefeat94]

Currently the docs only explain Ledger with Siacentral.
https://docs.sia.tech/integrations/using-the-sia-ledger-nano-app-sia-central

It should be extended to include instructions for Walletd.

[CtrlAltDefeat94]

https://docs.sia.tech/integrations/using-the-sia-ledger-nano-app-sia-central?q=ledger#ledger-nano-s

The Nano S is no longer supported and replaced by the Nano S Plus.