Point to local copy of developer docs

We now have a copy of the developer docs on the website, point to them
rather than developer.apertis.org (which should soon be redirected).

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/_index.md

@@ -76,4 +76,4 @@ Apertis provides a number of pre-built images for Intel (64-bit) and ARM (32-bit
 * [Design documents]( {{< ref "designs" >}} )
 * [Concept designs]( {{< ref "concepts" >}} )
 * [Supported APIs]( {{< ref "apis.md" >}} )
-* [App developer portal](https://developer.apertis.org)
+* [Developer Guide]( {{< ref "developer/_index.md" >}} )