echo, cat: add stdin example

pages/common/echo.md

@@ -27,3 +27,7 @@
 - Print the exit status of the last executed command (Note: In Windows Command Prompt and PowerShell the equivalent commands are `echo %errorlevel%` and `$lastexitcode` respectively):
 
 `echo $?`
+
+- Pass text to another program through `stdin`:
+
+`echo "{{Hello World}}" | {{program}}`

pages/linux/cat.md

@@ -26,3 +26,7 @@
 - Display non-printable and whitespace characters (with `M-` prefix if non-ASCII):
 
 `cat {{[-vte|--show-nonprinting -t -e]}} {{path/to/file}}`
+
+- Pass file contents to another program through `stdin`:
+
+`cat {{path/to/file}} | {{program}}`