Add tree command docs.

pages/common/tree.md

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+# tree
+
+> Show the contents of the current directory as a tree.
+
+- Show files and directories with a depth of 'num'
+
+`tree -L {{num}}`
+
+- Show directories only
+
+`tree -d`
+
+- Show hidden files too
+
+`tree -a`
+
+- Print human readable size of files
+
+`tree -h`