Customized 404 pages

If the user arrives to a page that does not exist, the erver will show a 404 Not Found error.

In some systems, such as GitHub pages, we can customize the content of the 404 -page by creating a file called 404.html.

mdBook automatically generates one for us, but if we create a file called 404.md in our source directory, then we can fully control the content of this page. See the 404 page of this book.

It does not need to be listed in the SUMMARY.md file.