Reinforcing docs on /core-concepts/keys-and-signatures

### Problem

From recent conversations about keys and accounts, we can strengthen the content on this page:
https://docs.hedera.com/hedera/core-concepts/keys-and-signatures

### Solution

The page https://docs.hedera.com/hedera/core-concepts/keys-and-signatures should be updated to include information about:
- `ecrecover` considerations (won't work for all ECDSA-keyed accounts, only for a specific subset)
- **Key rotation**: What is it? Why is it possible on Hedera? Considerations (when to rotate keys, considerations)
- More extensive FAQ (DevRel, Product, and DAs can help with FAQ). Will follow up directly.

cc: @theekrystallee @SimiHunjan

### Alternatives

_No response_