build: modularize the gulp file to be easier to maintain (#14259)

This is a precursor to bringing in some of the gulp tasks from angular.io,
which could lead to the gulpfile becoming unwieldy.

tools/gulp-tasks/docs-app.js

@@ -0,0 +1,3 @@
+module.exports = (gulp) => () => {
+  gulp.src('docs/src/**/*').pipe(gulp.dest('dist/docs'));
+};