README.mdx

[jonathantneal]

Octavo could switch from doc.mdx to README.mdx.

GitHub supports README.mdx files and gives them the same treatment as README.md files when viewing a directory. NPM also supports README.mdx files.

In both cases, they are displayed static, meaning the react components themselves are not hydrated. Self-closing elements, like the knobs component, would not be rendered at all in the static view, which may actually be ideal.

Using README.mdx seems like a better fallback pattern, and it would result in less repetition than having both docs.mdx and README.mdx files.