docs: Specify MDX React version

[pointlessrapunzel]

It has been changed in the README and the Gatsby Getting Started page, but not in the main Getting Started page, so this fixes it.

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Name	Status	Preview	Updated
theme-ui	✅ Ready (Inspect)	Visit Preview	May 16, 2022 at 2:46PM (UTC)

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[lachlanjc]

Thank you!

[nicobrinkkemper]

Just FYI, mdx@1 will not work with React 18 out of the box

Found: react@17.0.2
npm WARN node_modules/react
npm WARN   peer react@"^16.13.1 || ^17.0.0" from @mdx-js/react@1.6.22

[hasparus]

Just FYI, mdx@1 will not work with React 18 out of the box

Found: react@17.0.2
npm WARN node_modules/react
npm WARN   peer react@"^16.13.1 || ^17.0.0" from @mdx-js/react@1.6.22

Did you encounter any problems apart from the peer dependency warning?

[nicobrinkkemper]

Did you encounter any problems apart from the peer dependency warning?

I'm not too sure if I use the MDX functionality, but so far it works. I used the override property in package.json to mitigate the error/warning.

[hasparus]

🚀 PR was released in v0.15.0 🚀