chore(docs): Add "ESM and Gatsby"

[LekoArts]

Description

This adds a new "ES Modules (ESM) and Gatsby" document at /docs/how-to/custom-configuration/es-modules/. ESM support was released in 5.3 (https://www.gatsbyjs.com/docs/reference/release-notes/v5.3/#es-modules-esm-in-gatsby-files) but we never added the docs for it.

This PR also does:

• Update the README of gatsby-plugin-mdx to use ESM
• Update using-mdx example to use ESM

Documentation

Updating gatsby-plugin-mdx README makes sense as otherwise the usage is pretty limited.

Tests

I ran using-mdx locally. The README of gatsby-plugin-mdx is based on that.