fincore: add page (#15362)

* Create fincore.md

* Update fincore.md

pages/linux/fincore.md

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+# fincore
+
+> Display how much cache memory a file is taking.
+> More information: <https://manned.org/fincore>.
+
+- Display cache details for a file:
+
+`fincore {{path/to/file}}`
+
+- Display all possible data columns:
+
+`fincore --output-all {{path/to/file}}`