Group pages within their own directory

Add `pages` as it's own content type, and move all page content into
the appropriate directory.

Any references to `layout` in the YAML front matter has also been
removed as it's implied by the content type. Except for `about.md`
which does explicity use a different layout.

source/_pages/terms.md

@@ -0,0 +1,7 @@
+---
+title: Development Terms
+use: [posts]
+---
+# Development Terms
+
+TODO