Add relay configuration examples for popular Nostr relays

[singularityjason]

Description

Expand the documentation with configuration examples for different Nostr relay setups — self-hosted, paid relays, and relay selection guidance.

Acceptance Criteria

• Document how to configure custom relays in ~/.lightning-memory/config.json
• List 5-10 recommended relays with their characteristics (speed, reliability, paid vs free)
• Add a "Choosing Relays" section to the README or a separate docs/relays.md

Context

The default relays (Damus, nos.lol, relay.nostr.band) work, but users may want to use paid relays for reliability or self-host for privacy. This is a documentation-only contribution.

[P-r-e-m-i-u-m]

Hi @singularityjason, I would like to work on this issue. Could you please assign it to me?