Rename diagram.md to diagram.html, because it is HTML

Also use plain `include` directive

Author: abitrolly

docs/diagram.md renamed to docs/diagram.html

@@ -18,7 +18,7 @@ How it works
 This diagram summarises the steps that cibuildwheel takes on each platform to build your package's wheels.
 
 {%
-   include-markdown "diagram.md"
+   include "diagram.html"
 %}
 
 This isn't exhaustive, for a full list of the things cibuildwheel can do, check the [options](options.md) page.