[Doc Fix]: Remove google-generativeai import patterns from introduction.md, README, and usage guides

[YuuGR1337]

Fixes #88

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[rosspeili]

Thanks for taking the time to work on #88, we appreciate the contribution.

We can’t merge this PR as it stands. The diff only adds four lines to README, and they don’t complete the migration: it inserts from google import genai, closes the code block, adds meta text about what a “full patch would be,” and leaves the old google.generativeai / GenerativeModel example in place. That isn’t a runnable example for contributors.

#88 also requires updates to docs/introduction.md, docs/usage/gemini.md, and the listed skill docs, none of those are touched here.

Another contributor is already addressing #88 in #92. I will close this PR to avoid duplicate work, and feel free to pick another open issue. I see that you have an open PR in ARPAHLS/OPSIE, which needs your attention. Thanks again.