Update documentation relative links to include extension

[gingerrific]

The relative links within the docs for api/formik and [guides/validation](https://formik.org/docs/guides/validation) did not include a .md file extension and the code in rehype-docs.js was expecting one. Due to the lack of extension on these relative doc files, the call to basePath.lastIndexOf('.'); is getting the "dot" in the relative path ../. Simply adding .md to these relative links correctly fixes the routing in the notion documentation

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