feat: Cohere Docs Theme

[abvthecity]

This PR forks the default theme to construct a custom theme for Cohere.

Click here for preview - compare to Cohere's dashboard

I've decided to make this a big PR because it creating the Cohere theme required a lot of rethinking around architecture. We'll rely on in-depth smoke-testing (including mobile/responsive layouts) to ensure there are no regressions.

Also in this PR

• more global state has been moved into Jotai atoms. Jotai is well-suited for Fern Docs because it support SSR hydration, and enables complex selector logic that also reduces rerendering (greatly improved performance). This makes it possible to reason about responsive layout logic, especially wrt whether or not there's a header offset that needs to be accounted for when triggering scrollTo(), which was needed for cohere's theme.
• a lot more css classnames are now set so that the stylesheet can be controlled across separate themes. This will make it possible to generate more themes in the future.
• mobile sidebar is fixed on changelog pages (it's completely rewritten)
• FernLink is more resilient to mistakes; fixes table of contents bugs
• refactored rehypeLayout to use react components instead
• lots of optimizations as a result of using jotai— excess re-rendering cycles on endpoint pages has been eliminated.

Before	After
Screen.Recording.2024-07-06.at.12.45.42.PM.mov	Screen.Recording.2024-07-06.at.12.46.53.PM.mov

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