doc: make building with ninja more discoverable

Building with ninja would speed up the build
We have the documenation about building with ninja,
but not easily discoverable.
And present this early to the contributors.

BUILDING.md

@@ -273,6 +273,10 @@ $ ./configure
 $ make -j4
 ```
 
+We can speed up the builds by using [Ninja](https://ninja-build.org/). For more
+information, see
+[Building Node.js with Ninja](doc/contributing/building-node-with-ninja.md).
+
 The `-j4` option will cause `make` to run 4 simultaneous compilation jobs which
 may reduce build time. For more information, see the
 [GNU Make Documentation](https://www.gnu.org/software/make/manual/html_node/Parallel.html).

doc/contributing/pull-requests.md

@@ -244,6 +244,10 @@ test suite. To run the tests (including code linting) on Unix / macOS:
 ./configure && make -j4 test
 ```
 
+We can speed up the builds by using [Ninja](https://ninja-build.org/). For more
+information, see
+[Building Node.js with Ninja](building-node-with-ninja.md).
+
 And on Windows:
 
 ```text