Create build-a-poem-generator-with-google-gemini.mdx

Adding the poem generator PT

Closes #issue

[x] My project tutorial topic has already been approved by the Codédex team.
[x] My project tutorial file renders properly in mdx.
[x] My .mdx file follows the project template, as well as what's recommended in the content guidelines.
[x] The content written is my own, and not plagiarized from other sources.
[x] All code written is thoroughly functional and commented (or documented).
[x] I have already completed a self-review of the project tutorial.
[x] I will comply with changes made or recommended by the Codédex team about my project tutorial.

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