docs: use the Motoko identity-attributes library for II attribute verification (#286)

## Summary
Reframes the Internet Identity attribute flow in the authentication
guide around the two-method protocol (`_internet_identity_sign_in_start`
/ `_internet_identity_sign_in_finish`): the `mo:identity-attributes`
mixin provides it in Motoko, hand-written in Rust (no Rust library yet).
One frontend works against either backend.

## Changes
- **Frontend**: nonce, sign-in, and `requestAttributes` run in parallel
(nonce passed as a promise); requests `name` + `verified_email`; checks
the `{ ok }`/`{ err }` result.
- **Motoko tab**: `include IdentityAttributes({ onVerified })` plus
`mops.toml` and `icp.yaml` env vars (`trusted_attribute_signers`,
`frontend_origins`, optional `trusted_sso_domains`).
- **Rust tab**: hand-implements the two methods with the full manual
verification, mirroring the library.
- Storing-the-nonce tip, common-mistakes entry, and the upstream footer
updated (adds `dfinity/motoko-identity-attributes`).

## Verification
- The edited `internet-identity.mdx` compiles cleanly via `@mdx-js/mdx`.
- Full `astro build` deferred to CI (the local sandbox lacked the
`.sources` submodules; all sandbox build errors were on other pages,
none on this file).

Author: sea-snake