Define docs target as a toolchain

This change creates a toolchain that configures the docs target and adds
it to the default build.

Change-Id: Ib2a8387253eef809f94411780174377e0d23e1a9
diff --git a/pw_cpu_exception/BUILD.gn b/pw_cpu_exception/BUILD.gn
index d90b1c7..137fd84 100644
--- a/pw_cpu_exception/BUILD.gn
+++ b/pw_cpu_exception/BUILD.gn
@@ -17,6 +17,7 @@
 
 import("$dir_pw_build/facade.gni")
 import("$dir_pw_docgen/docs.gni")
+import("backend.gni")
 config("default_config") {
   include_dirs = [ "public" ]
 }