<, >, |, $: add page (#13846)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/$.md

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+# Dollar sign
+
+> Expand a bash variable.
+> More information: <https://gnu.org/software/bash/manual/bash.html#Shell-Variables>.
+
+- Print a variable:
+
+`echo ${{VARIABLE}}`
+
+- Print the exit status of the previous command:
+
+`echo $?`
+
+- Print a random number between 0 and 32767:
+
+`echo $RANDOM`
+
+- Print one of the prompt strings:
+
+`echo ${{PS1|PS2|PS3|PS4}}`
+
+- Expand with the output of `command` and run it. Same as enclosing `command` in backtics:
+
+`$({{command}})`