- Add vendor/bundle into .gitignore template (#32930)

Summary:
Ref #32303 (comment)

Recently, Gemfile & .ruby-version have been introduced but local vendor artifacts have not been correctly added into gitignore, see #32303

## Changelog

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[General] [Added] - Add `vendor/bundle` into .gitignore template

Pull Request resolved: #32930

Test Plan: N / A

Reviewed By: cortinico

Differential Revision: D33688116

Pulled By: yungsters

fbshipit-source-id: 787b2b90da1e9f32f0a9e754741affbda443b3e8

template/_gitignore

@@ -56,5 +56,6 @@ buck-out/
 # Bundle artifact
 *.jsbundle
 
-# CocoaPods
+# Ruby / CocoaPods
 /ios/Pods/
+/vendor/bundle/