pg_walsummary: add page

pages/common/pg_walsummary.md

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+# pg_walsummary
+
+> Print contents of WAL summary files.
+> More information: <https://www.postgresql.org/docs/current/app-pgwalsummary.html>.
+
+- Convert a WAL summary file to text:
+
+`pg_walsummary {{path/to/file}}`
+
+- Print one line per individual modified block (rather than ranges):
+
+`pg_walsummary {{[-i|--individual]}} {{path/to/file}}`
+
+- Suppress normal output (only errors):
+
+`pg_walsummary {{[-q|--quiet]}} {{path/to/file}}`