docs/gatsby-node.js

const path = require('path')
const { createFilePath } = require('gatsby-source-filesystem')

exports.onCreateNode = ({ node, getNode, actions }) => {
    const { createNodeField } = actions

    if (node.internal.type === 'MarkdownRemark') {
        const fileNode = getNode(node.parent)
        const parsedFilePath = path.parse(fileNode.relativePath)
        const slug = createFilePath({ node, getNode, basePath: 'content' })
        const section = parsedFilePath.dir

        createNodeField({
            node,
            name: 'slug',
            value: slug
        })

        createNodeField({
            node,
            name: 'section',
            value: section
        })
    }
}

exports.createPages = ({ graphql, actions }) => {
    const { createPage } = actions

    return new Promise((resolve, reject) => {
        resolve(
            graphql(
                `
                    {
                        allMarkdownRemark {
                            edges {
                                node {
                                    fields {
                                        slug
                                        section
                                    }
                                }
                            }
                        }
                    }
                `
            ).then(result => {
                if (result.errors) {
                    /* eslint-disable-next-line no-console */
                    console.log(result.errors)
                    reject(result.errors)
                }

                const posts = result.data.allMarkdownRemark.edges
                const docTemplate = path.resolve('./src/templates/Doc.jsx')

                // Create Doc pages
                posts.forEach(post => {
                    createPage({
                        path: `${post.node.fields.slug}`,
                        component: docTemplate,
                        context: {
                            slug: post.node.fields.slug,
                            section: post.node.fields.section
                        }
                    })
                })

                resolve()
            })
        )
    })
}