Secrets docs: review follow-up + MPC keyshare storage

[emostov]

Stacked on #764.

Summary

• Apply Secrets review feedback and rename the API key storage page and icons.
• Add the MPC keyshare storage solution page, navigation, cards, and icons.

Turnkey treats MPC keyshares as opaque byte blobs: it does not parse, validate, reconstruct, or participate in MPC signing.

Verification

• npx prettier solutions/key-management/api-key-storage.mdx
• Targeted internal-link validation
• Plaintext-claim grep
• git diff --check