add doc for building examples with cmake build system

change ci matrix build:
- github build make/cmake one per family on push only
- circicle do full cmake build for all toolchain (missing rx-gcc)

.circleci/config.yml

@@ -19,13 +19,12 @@ jobs:
             echo "MATRIX_JSON=$MATRIX_JSON"
 
             BUILDSYSTEM_TOOLCHAIN=(
+              "cmake aarch64-gcc"
               "cmake arm-clang"
+              "cmake arm-gcc"
               "cmake esp-idf"
-              "make aarch64-gcc"
-              "make arm-gcc"
-              "make msp430-gcc"
-              "make riscv-gcc"
-              "make rx-gcc"
+              "cmake msp430-gcc"
+              "cmake riscv-gcc"
             )
 
             # only build IAR if not forked PR, since IAR token is not shared