Add custom 404 page

Author: afh

source/404.md

@@ -0,0 +1,14 @@
+---
+title: Not Found
+---
+
+# Page not found
+
+Unfortunately the page you are looking for cannot be found.
+
+Possibly the page moved to another location, or the link or URL was mistyped.
+
+Please use the navigation above to browse other content available on this website.
+
+If you believe there really should be a page under this URL please
+[create an issue](https://git.ledger-cli.org/ledger.github.io/issues/new).