fix(gatsby): handle /404/, /404, 404/ or 404 page paths (#15615)

Author: pieh

docs/docs/add-404-page.md

@@ -3,7 +3,7 @@ title: "Adding a 404 Page"
 ---
 
 To create a 404 page create a page whose path matches the regex
-`/404*`. Most often you'll want to create a React component page at
+`^\/?404\/?$` (`/404/`, `/404`, `404/` or `404`). Most often you'll want to create a React component page at
 `src/pages/404.js`.
 
 Gatsby ensures that your 404 page is built as `404.html` as many static hosting

packages/gatsby/src/internal-plugins/prod-404/gatsby-node.js

@@ -3,7 +3,7 @@ exports.onCreatePage = ({ page, store, actions }) => {
   // Copy /404/ to /404.html as many static site hosts expect
   // site 404 pages to be named this.
   // https://www.gatsbyjs.org/docs/add-404-page/
-  if (!created404 && page.path === `/404/`) {
+  if (!created404 && /^\/?404\/?$/.test(page.path)) {
     actions.createPage({
       ...page,
       path: `/404.html`,