xml: add pages (incomplete: sub-command templates only)

pages/common/xml.md

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+# xml
+
+> XMLStarlet toolkit: Query / Edit / Check / Transform XML documents.
+> Subcommands: `edit`, `select`, `transform`, `validate`, `format`, `elements`, `canonic`, `list`, `escape`, `unescape`, `pyx`, or `p2x`.
+> More information: <https://http://xmlstar.sourceforge.net/doc/xmlstarlet.pdf>.
+
+- Execute a subcommand:
+
+`xml {{subcommand}} {{options}} {{xml-file-or-uri}}`
+
+- Execute a subcommand, redirecting output to a file:
+
+`xml {{subcommand}} {{options}} {{xml-file-or-uri}} >{{path/to/output}}`
+
+- Display help for a subcommand:
+
+`xml {{subcommand}} --help`
+
+- Display general help:
+
+`xml --help`
+
+- Display version:
+
+`xml --version`