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/content/examples/.gitignore

@@ -18,6 +18,7 @@
 **/src/karma.conf.js
 **/.angular-cli.json
 **/.editorconfig
+**/.gitignore
 **/angular.json
 **/tsconfig.json
 **/bs-config.e2e.json