flash: add page (#4712)

pages/linux/flash.md

@@ -0,0 +1,24 @@
+# flash
+
+> Flash cards in the terminal.
+> More information: <https://github.com/tallguyjenks/fla.sh>.
+
+- Open a menu of available flashcard decks for selection:
+
+`flash`
+
+- Display the program version:
+
+`flash -v`
+
+- Display information about the flashcard system:
+
+`flash -i`
+
+- Display a list of available commands:
+
+`flash -h`
+
+- Change the previewer from default `bat` to `cat`:
+
+`flash -p {{cat}}`