Tutorial: Step 0 missing

[jablanco-cg]

In the tutorial, a "Step 0" is mentioned several times in the other steps, mainly for imports and env keys setup, but there is no Step 0 section in the document.

[koverholt]

Thanks for the bug report and for letting us know! We've fixed this in google/adk-docs#28 and google/adk-docs#35, so this should be resolved and published in the latest docs now.