Allow md As An Extension With gatsby-mdx

The gatsby-mdx plugin allows you to create pages in a Gatsby project using .mdx files. If you prefer the .md extension on your markdown files, then you can adjust the plugin options to allow that.

// gatsby-config.js
plugins: [
  {
    resolve: `gatsby-mdx`,
    options: {
      extensions: [".mdx", ".md"]
    }
  }
]

This tells gatsby-mdx to recognize both .mdx and .md extensions when processing files.