DVC: add page including sub-commands freeze, unfreeze and gc

pages/common/dvc-gc.md

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+# dvc gc
+
+> Remove unused files and directories from cache or remote storage.
+> More information: <https://dvc.org/doc/command-reference/gc>.
+
+- Garbage collect from cache, keeping only version referenced by current workspace:
+
+`dvc gc --workspace`
+
+- Garbage collect from cache, keeping only version referenced by branch/tags/all_commits:
+
+`dvc gc --all-branches --all-tags --all-commits`
+
+- Garbage collect from cache, including default cloud remote storage (if set):
+
+`dvc gc --all-commits --cloud`
+
+- Garbage collect from cache, including specific cloud remote storage:
+
+`dvc gc --all-commits --cloud -r {{remote_name}}`