DVC: add page including sub-commands checkout, init, add

pages/common/dvc-add.md

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+# dvc add
+
+> Adds changed files to the index.
+> More information: <https://dvc.org/doc/command-reference/add>.
+
+- Add a single target file to the index:
+
+`dvc add {{path/to/file}}`
+
+- Add target directory to the index:
+
+`dvc add {{path/to/directory}}`
+
+- Recursively add all files in target directory:
+
+`dvc add --recursive {{path/to/directory}}`
+
+- Add target file with custom .dvc file name
+
+`dvc add --file custom_name.dvc {{path/to/file}}`