doc,cmake: make find_package(Zephyr...) REQUIRED

This provides a better error message when building with CMake and
forgetting ZEPHYR_BASE or not registering Zephyr in the CMake package
registry. See parent commit for more details (split from parent for
better readability).

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 6827a37..4db1ce4 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -4,7 +4,7 @@
 project(Zephyr-Kernel-Doc LANGUAGES)
 
 set(NO_BOILERPLATE TRUE)
-find_package(Zephyr HINTS $ENV{ZEPHYR_BASE} ..)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE} ..)
 
 # Find west to (optionally) process modules for Kconfig
 find_program(