Merge pull request #544 from stan-dev/feature/404-page

Add 404 page

Author: WardBrian

docs/404.html

@@ -0,0 +1,29 @@
+---
+title: "Stan Documentation"
+output:
+  bookdown::html_document2:
+    css: stan-manual.css
+    pandoc_args: ["--syntax-definition","./stan.xml"]
+    highlight: tango
+    split_by: "section"
+    number_sections: false
+
+---
+<br/>
+
+### Page not found
+
+The page you requested cannot be found (perhaps it was moved or renamed).
+
+You may want to try searching to find the page's new location, or use the table of contents to find the page you are looking for.
+
+[Return to docs home.](https://mc-stan.org/docs)
+
+
+----
+
+```stan
+        reject("This page cannot be found");
+```
+
+<!-- rendered with bookdown::render_book("404.Rmd") -->