minor symfony#3754 ignore more files and directories which are created when building the documentation (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

ignore more files and directories which are created when building the documentation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

5f5e8a5 ignore more files and directories which are created when building the documentation

Author: weaverryan

.gitignore

@@ -1 +1,8 @@
-_exts
+/_build
+/bundles/DoctrineFixturesBundle
+/bundles/DoctrineMigrationsBundle
+/bundles/DoctrineMongoDBBundle
+/bundles/SensioFrameworkExtraBundle
+/bundles/SensioGeneratorBundle
+/cmf
+/_exts