docs: update Tempo WebAuthn examples

[suyua9]

Summary

• remove the stale KeyManager import from Tempo zone docs examples
• switch the examples to call webAuthn() directly
• keep the existing chain/client setup unchanged

Context

The Tempo docs examples still referenced KeyManager.localStorage(), but KeyManager is no longer exported from wagmi/tempo. This keeps the examples aligned with the current public API.

Tests

• git diff --check
• Not run: documentation-only change; local commit hook could not run because this worktree has no installed node_modules (biome: command not found)

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