feat(docs): Expand stub for "Creating slugs for pages" (#8732)

* Add content to Creating slugs for pages

* Shift content to doc format

docs/docs/creating-slugs-for-pages.md

@@ -2,7 +2,48 @@
 title: Creating slugs for pages
 ---
 
-This is a stub. Help our community expand it.
+The logic for creating slugs from file names can get tricky, the `gatsby-source-filesystem` plugin ships with a function for creating them.
 
-Please use the [Gatsby Style Guide](/docs/gatsby-style-guide/) to ensure your
-pull request gets accepted.
+## Install
+
+`npm install --save gatsby-source-filesystem`
+
+## Create slugs in gatsby-node.js
+
+Add your new slugs directly onto the `MarkdownRemark` nodes. Any data you add to nodes is available to query later with GraphQL.
+
+To do so, you'll use a function passed to our API implementation called [`createNodeField`](/docs/bound-action-creators/#createNodeField). This function allows you to create additional fields on nodes created by other plugins.
+
+```javascript{3,4,6-11}:title=gatsby-node.js
+const { createFilePath } = require(`gatsby-source-filesystem`)
+
+exports.onCreateNode = ({ node, getNode, actions }) => {
+  const { createNodeField } = actions
+  if (node.internal.type === `MarkdownRemark`) {
+    const slug = createFilePath({ node, getNode, basePath: `pages` })
+    createNodeField({
+      node,
+      name: `slug`,
+      value: slug,
+    })
+  }
+}
+```
+
+## Query created slugs
+
+Open refresh Graph_i_QL, then run this GraphQL query to see all your slugs:
+
+```graphql
+{
+  allMarkdownRemark {
+    edges {
+      node {
+        fields {
+          slug
+        }
+      }
+    }
+  }
+}
+```