docs(gatsby-source-wordpress): Remove lodash + bluebird + fix Promise…

… hell (#11739)

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## Description

This commit:
- Removes lodash in favor of Array.forEach
- Bluebird in favor of the native Promise object
- Avoids nested thenables by using Promise.all()

I've been using this code in my builds and it's been working fine for whatever that worth :)

packages/gatsby-source-wordpress/README.md

@@ -728,8 +728,6 @@ and also your `wordpress-source-plugin` options from `gatsby-config.js`. To lear
 ## Site's `gatsby-node.js` example
 
 ```javascript
-const _ = require(`lodash`)
-const Promise = require(`bluebird`)
 const path = require(`path`)
 const slash = require(`slash`)
 
@@ -739,101 +737,82 @@ const slash = require(`slash`)
 // create pages.
 // Will create pages for WordPress pages (route : /{slug})
 // Will create pages for WordPress posts (route : /post/{slug})
-exports.createPages = ({ graphql, actions }) => {
+exports.createPages = async ({ graphql, actions }) => {
   const { createPage } = actions
-  return new Promise((resolve, reject) => {
-    // The “graphql” function allows us to run arbitrary
-    // queries against the local WordPress graphql schema. Think of
-    // it like the site has a built-in database constructed
-    // from the fetched data that you can run queries against.
-
-    // ==== PAGES (WORDPRESS NATIVE) ====
-    graphql(
-      `
-        {
-          allWordpressPage {
-            edges {
-              node {
-                id
-                slug
-                status
-                template
-              }
-            }
+
+  // The “graphql” function allows us to run arbitrary
+  // queries against the local WordPress graphql schema. Think of
+  // it like the site has a built-in database constructed
+  // from the fetched data that you can run queries against.
+  const result = await graphql(`
+    {
+      allWordpressPage {
+        edges {
+          node {
+            id
+            link
+            status
+            template
           }
         }
-      `
-    )
-      .then(result => {
-        if (result.errors) {
-          console.log(result.errors)
-          reject(result.errors)
+      }
+      allWordpressPost {
+        edges {
+          node {
+            id
+            link
+            status
+            template
+            format
+          }
         }
+      }
+    }
+  `)
 
-        // Create Page pages.
-        const pageTemplate = path.resolve("./src/templates/page.js")
-        // We want to create a detailed page for each
-        // page node. We'll just use the WordPress Slug for the slug.
-        // The Page ID is prefixed with 'PAGE_'
-        _.each(result.data.allWordpressPage.edges, edge => {
-          // Gatsby uses Redux to manage its internal state.
-          // Plugins and sites can use functions like "createPage"
-          // to interact with Gatsby.
-          createPage({
-            // Each page is required to have a `path` as well
-            // as a template component. The `context` is
-            // optional but is often necessary so the template
-            // can query data specific to each page.
-            path: `/${edge.node.slug}/`,
-            component: slash(pageTemplate),
-            context: {
-              id: edge.node.id,
-            },
-          })
-        })
-      })
-      // ==== END PAGES ====
-
-      // ==== POSTS (WORDPRESS NATIVE AND ACF) ====
-      .then(() => {
-        graphql(
-          `
-            {
-              allWordpressPost {
-                edges {
-                  node {
-                    id
-                    slug
-                    status
-                    template
-                    format
-                  }
-                }
-              }
-            }
-          `
-        ).then(result => {
-          if (result.errors) {
-            console.log(result.errors)
-            reject(result.errors)
-          }
-          const postTemplate = path.resolve("./src/templates/post.js")
-          // We want to create a detailed page for each
-          // post node. We'll just use the WordPress Slug for the slug.
-          // The Post ID is prefixed with 'POST_'
-          _.each(result.data.allWordpressPost.edges, edge => {
-            createPage({
-              path: `/${edge.node.slug}/`,
-              component: slash(postTemplate),
-              context: {
-                id: edge.node.id,
-              },
-            })
-          })
-          resolve()
-        })
-      })
-    // ==== END POSTS ====
+  // Check for any errors
+  if (result.errors) {
+    throw new Error(result.errors)
+  }
+
+  // Access query results via object destructuring
+  const { allWordpressPage, allWordpressPost } = result.data
+
+  // Create Page pages.
+  const pageTemplate = path.resolve(`./src/templates/page.js`)
+  // We want to create a detailed page for each
+  // page node. We'll just use the WordPress Slug for the slug.
+  // The Page ID is prefixed with 'PAGE_'
+  allWordpressPage.edges.forEach(edge => {
+    // Gatsby uses Redux to manage its internal state.
+    // Plugins and sites can use functions like "createPage"
+    // to interact with Gatsby.
+    createPage({
+      // Each page is required to have a `path` as well
+      // as a template component. The `context` is
+      // optional but is often necessary so the template
+      // can query data specific to each page.
+      path: `/${edge.node.slug}/`,
+      component: slash(pageTemplate),
+      context: {
+        id: edge.node.id,
+      },
+    })
+  })
+
+  // Create Post pages
+  const postTemplate = path.resolve(`./src/templates/post.js`)
+  // We want to create a detailed page for each
+  // post node. We'll just use the WordPress Slug for the slug.
+  // The Post ID is prefixed with 'POST_'
+  allWordpressPost.edges.forEach(edge => {
+    createPage({
+      path: `/${edge.node.slug}/`,
+      component: slash(postTemplate),
+      context: {
+        id: edge.node.id,
+      },
+    })
   })
 }
 ```