xmlstarlet: add page (#9952)

* xmlstarlet: add page
---------

Co-authored-by: Jack Lin <blueskyson1401@gmail.com>

pages/common/xmlstarlet.md

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+# xmlstarlet
+
+> A commandline XML/XSLT toolkit.
+> Note: You will likely need to know XPath: <https://developer.mozilla.org/en-US/docs/Web/XPath>.
+> More information: <https://xmlstar.sourceforge.net/docs.php>.
+
+- Format an XML document and print to stdout:
+
+`xmlstarlet format {{path/to/file.xml}}`
+
+- XML document can also be piped from stdin:
+
+`{{cat path/to/file.xml}} | xmlstarlet format`
+
+- Print all nodes that match a given XPath:
+
+`xmlstarlet select --template --copy-of {{xpath}} {{path/to/file.xml}}`
+
+- Insert an attribute to all matching nodes, and print to stdout (source file is unchanged):
+
+`xmlstarlet edit --insert {{xpath}} --type attr --name {{attribute_name}} --value {{attribute_value}} {{path/to/file.xml}}`
+
+- Update the value of all matching nodes in place (source file is changed):
+
+`xmlstarlet edit --inplace --update {{xpath}} --value {{new_value}} {{file.xml}}`
+
+- Delete all matching nodes in place (source file is changed):
+
+`xmlstarlet edit --inplace --delete {{xpath}} {{file.xml}}`
+
+- Escape or unescape special XML characters in a given string:
+
+`xmlstarlet [un]escape {{string}}`
+
+- List a given directory as XML (omit argument to list current directory):
+
+`xmlstarlet ls {{path/to/directory}}`