po4a: add page (#6493)

pages/linux/po4a-gettextize.md

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+# po4a-gettextize
+
+> Convert a file to a PO file.
+> More information: <https://po4a.org/man/man1/po4a-gettextize.1.php>.
+
+- Convert a text file to PO file:
+
+`po4a-gettextize --format {{text}} --master {{path/to/master.txt}} --po {{path/to/result.po}}`
+
+- Get a list of available formats:
+
+`po4a-gettextize --help-format`
+
+- Convert a text file along with a translated document to a PO file (`-l` option can be provied multiple times):
+
+`po4a-gettextize --format {{text}} --master {{path/to/master.txt}} --localized {{path/to/translated.txt}} --po {{path/to/result.po}}`