Update Contributing doc with a note about CommonMark for MDX files

[paigecalvert]

We should make sure the Contributing doc has a note about CommonMark changes needed like escaping characters for .mdx files specifically (we use auto-detect so we don't need to do that on .md files). Can maybe just link to markdown and JSX interoperability: https://docusaurus.io/docs/markdown-features/react#markdown-and-jsx-interoperability