DVC: add pages for sub commands commit, config, dag

pages/common/dvc-config.md

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+# dvc config
+
+> Low level command to manage custom configuration options for dvc repositories.
+> These configurations can be project/local/global/system level.
+> More information: <https://dvc.org/doc/command-reference/config>.
+
+- Get default remote name:
+
+`dvc config core.remote`
+
+- Set project default remote:
+
+`dvc config core.remote {{remote_name}}`
+
+- Unset project default remote:
+
+`dvc config --unset core.remote`
+
+- Get project config value for a key:
+
+`dvc config key`
+
+- Set project level config value for a key:
+
+`dvc config key value`
+
+- Unset project level config value for a key:
+
+`dvc config --unset key`
+
+- Set local/global/system level config:
+
+`dvc config --local/global/system key value`