Pass docs goal to kati

The docs goal affects makefile parsing (android_soong_config_variables),
so it needs to be passed to kati to work properly.

Bug: 251762621
Test: TARGET_PRODUCT=sdk m docs dist (in prebuilt branch)
Change-Id: Ifa8e1e8e3a239434ac40a3edb213ed6344ceb424
diff --git a/core/ninja_config.mk b/core/ninja_config.mk
index e436b2c..2b5ceee 100644
--- a/core/ninja_config.mk
+++ b/core/ninja_config.mk
@@ -25,6 +25,7 @@
 	cts \
 	custom_images \
 	dicttool_aosp \
+	docs \
 	eng \
 	oem_image \
 	online-system-api-sdk-docs \