fix(docs-infra): include .gitignore file in CLI-based docs examples (#38173)

Previously, the `.gitignore` file that is part of the boilerplate files
for CLI-based docs examples (located in
`aio/tools/examples/shared/boilerplate/cli/`) was not added to the
example projects, because it was not included in the boilerplate file
list in `example-boilerplate.js`.

This commit fixes it by adding the `.gitignore` file to the list. This
ensures that docs examples more closely match CLI-generated projects.

PR Close #38173

aio/tools/examples/example-boilerplate.spec.js

@@ -10,7 +10,7 @@ describe('example-boilerplate tool', () => {
     const sharedDir = path.resolve(__dirname, 'shared');
     const sharedNodeModulesDir = path.resolve(sharedDir, 'node_modules');
     const BPFiles = {
-      cli: 20,
+      cli: 21,
       i18n: 3,
       universal: 2,
       systemjs: 7,