Reference dirs builtin in pushd/popd (#5759)

pages/common/pushd.md

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+# pushd
+
+> Place a directory on a stack so it can be accessed later.
+> See also `popd` to switch back to original directory and `dirs` to display directory stack contents.
+
+- Switch to directory and push it on the stack:
+
+`pushd {{directory}}`
+
+- Switch first and second directories on the stack:
+
+`pushd`
+
+- Rotate stack by making the 5th element the top of the stack:
+
+`pushd +4`