tlmgr-shell: add page (#6572)

pages/common/tlmgr-shell.md

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+# tlmgr shell
+
+> Start an interactive shell of the native TeX Live manager.
+> More information: <https://www.tug.org/texlive/tlmgr.html>.
+
+- Start an interactive shell of `tlmgr`:
+
+`tlmgr shell`
+
+- Run any `tlmgr` sub-command in the interactive shell:
+
+`{{sub_command}} {{arguments}}`
+
+- Quit the interactive shell:
+
+`quit`
+
+- List all TeX Live variables:
+
+`get`
+
+- Get the value of a TeX Live variable:
+
+`get {{variable}}`
+
+- Set the value of a TeX Live variable:
+
+`set {{variable}} {{value}}`
+
+- Restart the interactive shell:
+
+`restart`
+
+- Print the version of the current protocoll:
+
+`protocol`