IPNS Publisher Key Seed Phrases

This is a placeholder for writing down an IPIP with a spec for the scope below

## Scope

New document in https://specs.ipfs.tech/ipns/ that describes:

- producing a seed phrase at the time of IPNS key generation for backup purposes
- recreating IPNS publishing key from a backup seed phrase

MVP could be specific to ED25519 keys, as long it creates a convention and space for future IPIPs with other key types.

### Use cases

- import/export keys including air-gapped contexts and/or where PEM is not supported
- improving UX around website publishing (e.g. blogging platform asking user to write down seed phrases as offline backup, or asking to provide existing one)

### Prior art

- ed25519 backup/restore https://github.com/charmbracelet/melt
- crypto wallets: [BIP-32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki), [BIP-39](https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki)