mountpoint: add page (#2963)

pages/linux/mountpoint.md

@@ -0,0 +1,15 @@
+# mountpoint
+
+> Test if a directory is a filesystem mountpoint.
+
+- Check if a directory is a mountpoint:
+
+`mountpoint {{path/to/directory}}`
+
+- Check if a directory is a mountpoint without showing any output:
+
+`mountpoint -q {{path/to/directory}}`
+
+- Show major/minor numbers of a mountpoint's filesystem:
+
+`mountpoint --fs-devno {{path/to/directory}}`