docs: Add list of hardware supported by the Petitkernel

This list is taken from the current kernel configuration. The intent is
it will be updated as requests come in to add new hardware.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/doc/index.rst b/doc/index.rst
index 4bbc061..d4d9668 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -19,6 +19,7 @@
    introduction
    testing
    process/index
+   boot-devices
    versioning
    release-notes/index