add frontmatter documentation

Author: chargome

docs/contributing/pages/frontmatter.mdx

@@ -47,3 +47,13 @@ Much of the other functionality for pages is also driven via frontmatter, such a
 
 - [Redirects](../redirects/)
 - [Search](../search/)
+
+`nextPage` (`{ path: 'path/to/page', title: 'Page Title' }`)
+
+Overrides the next page shown in the bottom pagination navigation.
+
+`previousPage` (`{ path: 'path/to/page', title: 'Page Title' }`)
+
+Overrides the previous page shown in the bottom pagination navigation.
+
+