Cross-link e2ee.md from authentication.md and overview.md #5

The new `features/e2ee.md` page (added in #3) is reachable only from the sidebar. Add inbound links so users land there from the natural reading paths:

1. `features/authentication.md` — append a short section at the bottom of the Nostr Authentication block: "End-to-end encryption is a downstream capability of did:nostr — see End-to-End Encryption."
2. `features/overview.md` — add an E2EE row in the Authentication & Identity table directly after the Nostr NIP-98 row.

Small change; same scope as #3.