docs(framework-agnostic.mdx): remove getCssText from core import

[saurabhdaware]

In docs it is mentioned that we can import getCssText from @stitches/core but seems like it is not working. The getCssText is returned from createStitches API.

I checked the exports in the code and there too it doesn't seem like we're exporting getCssText directly.

Let me know if I misunderstood something! Thanks for building this!

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[percy507]

https://github.com/stitchesjs/stitches-site/blob/master/data/docs/installation.mdx?plain=1#L72

Same to @stitches/react package, the getCssText method is also not exported.