Update split keys quickstart to match charon behavior

[pinebit]

Update the "Migrate an Existing Validator" (split keys) quickstart and the CLI reference to match actual charon behavior:

• Add --cluster-dir=/opt/charon/cluster to the example docker command; without it the cluster is created in ./node*, contradicting the page's ./cluster tree and the zip step.
• Document that deposit-data-*.json files ARE re-created in split mode, signed with the existing validator keys using the provided withdrawal addresses, and add a warning not to submit them for already-activated validators. The page previously omitted these files entirely.
• Match the command output block to actual charon output (Created charon cluster, cluster dir path, deposit-data files).
• Extend the v1.8.0 note: manual keystore preparation is required when specifying an exact list of withdrawal or fee recipient addresses.
• Update the --split-existing-keys flag description in the CLI reference.

Note: the fee-recipient ordering note and the updated flag description correspond to ObolNetwork/charon#4577 and apply from the charon release containing it; the deposit data documentation is accurate for current releases as well.

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