Recommend `-G Ninja` in all in-tree documentation. (#15328)

We strongly recommend using https://ninja-build.org/. Other generators
typically work, but ninja is fast by default and is portable to all of
our supported platforms.

Found these with a few regex searches (mainly `cmake -B` and
`^\s*cmake.*-B`)

samples/static_library/README.md

diff --git a/samples/static_library/README.md b/samples/static_library/README.md
index 3b84bf2..4a38dfd 100644
--- a/samples/static_library/README.md
+++ b/samples/static_library/README.md
@@ -37,7 +37,7 @@
 for general instructions on building using CMake):
 
   ```shell
-  cmake -B ../iree-build/ \
+  cmake -B ../iree-build/ -G Ninja \
     -DCMAKE_BUILD_TYPE=RelWithDebInfo .
     -DIREE_BUILD_SAMPLES=ON \
     -DIREE_TARGET_BACKEND_DEFAULTS=OFF \