Docs: document the Umami API key field

[SandObserver]

Summary

• Add the Umami API key field to the Connections service table, with the version note and the fallback to a username and password.
• Change the Umami "needs" summary to "URL, website ID, API key or login".

Testing
npm run build and npm run check both pass. Text-only change to an existing table, no visual change.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

[cloudflare-workers-and-pages]

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