Partials are currently defined via MDX includes. This PR switches to pre-compile-time partials, which have a new syntax:
```
<$Partial path="path/to/file.mdx" />
```
## Rationale
This produces two improvements:
1. Partial substitution can occur in pipelines that don't use MDX compilation. For example, we can now do partial substitution before building the search index, so partial content will also be indexed.
2. After the App Router migration, the MDXProviders should've been deprecated, but were kept around for the sole reason of making partials work, and leading to us shipping unnecessary client-side code. We get a minor decrease in overall client bundle size (5.74 MB to 5.6 MB) by getting rid of the Providers.
## Breaking changes
Besides the change to partial syntax, the arguments are also less powerful than before because we are doing string substitution and don't have the full power of JS. Defining string variables is still possible (documented in the Contributing guide), and since that's all we actually do in practice, this shouldn't be too cumbersome. There is always the escape hatch of making a custom component for more complex content reuse cases.
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