docs: Update ecdsa messaging

[ty-swirldslabs]

Description:

This PR updates key type documentation across three pages to recommend ECDSA (secp256k1) for all new accounts and applications, reflecting its advantages for EVM and smart contract compatibility.

• Update keys-and-signatures.mdx to recommend ECDSA as the default key type, restructure the key type section with dedicated ECDSA and Ed25519 subsections, add accurate EVM alias derivation details, add a code example showing setECDSAKeyWithAlias() for account creation, expand the comparison table, and add new FAQ entries for key type selection and EVM alias explanation
• Update generate-a-new-key-pair.mdx to list ECDSA first and update all code examples to use ECDSA key generation methods, moving ED25519 to a secondary section
• Update import-an-existing-key.mdx to replace all ED25519 key import examples with ECDSA equivalents across Java, JavaScript, Go, and Rust

Notes for reviewer:

The EVM alias description on keys-and-signatures.mdx is taken verbatim from the HIP/protocol spec: the alias is the rightmost 20 bytes of the 32-byte Keccak-256 hash of the ECDSA public key, per the Ethereum Yellow Paper, with the recovery ID excluded. The code example uses setECDSAKeyWithAlias() to clarify that the alias must be explicitly set at account creation, it is not set automatically.

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[theekrystallee]

LGTM. thank you!