Tidy up guidelines

Move guidelines.md to be index for guilelines directory.

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

content/_index.md

@@ -69,7 +69,7 @@ Apertis provides a number of pre-built images for Intel (64-bit) and ARM (32-bit
 * [Source code](https://gitlab.apertis.org)
 * [Mailing lists](https://lists.apertis.org/)
 * [Set up your development environment]( {{< ref "/deployment_environment.md" >}} )
-* [Development guidelines]( {{< ref "/guidelines.md" >}} )
+* [Development guidelines]( {{< ref "/guidelines/_index.md" >}} )
 * [Concept designs]( {{< ref "/conceptdesigns.md" >}} )
 * [Supported APIs](https://developer.apertis.org/latest/apis.html)
 * [App developer portal](https://developer.apertis.org)