docs: fix broken links

[vdusart]

I fixed some broken links, but there are still a few left and I don't know what you want to do with them... I guess the doc isn't written yet :)

But just in case, here they are:

# In the README.md file

- [Getting Started - Framework](https://reactrouter.com/start/framework/installation)
- [Getting Started - Library](https://reactrouter.com/start/library/installation)
- [Upgrade from v6](https://reactrouter.com/upgrading/v6)
- [Upgrade from Remix](https://reactrouter.com/upgrading/remix)

Thanks for maintaining react-route :smile:

[changeset-bot[bot]]

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Latest commit: 288468ab7318be0226b6efe18697726bb9560370

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[remix-cla-bot[bot]]

Hi @vdusart,

Welcome, and thank you for contributing to React Router!

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[remix-cla-bot[bot]]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳