Add mpox/clade-I to manifest

[jameshadfield]

See https://github.com/nextstrain/mpox/pull/254 for context on the dataset itself.

The dataset itself is live (via this action run), this PR just adds it to the sidebar dropdown in Auspice.

• [x] Checks pass
• [x] Check if changes affect the resource index JSON revision -- The resource indexer should handle this appropriately regardless of these changes.