{greater, less}-than: add persistent file descriptor example (#17773)

* Update less-than.md and greater-than.md

pages/common/greater-than.md

@@ -26,3 +26,11 @@
 - Redirect `stderr` to `stdout` for piping them together:
 
 `{{command1}} 2>&1 | {{command2}}`
+
+- Open a persistent file descriptor:
+
+`exec {{3}}>{{path/to/file}}`
+
+- Write to a custom file descriptor:
+
+`{{echo text}} >&{{3}}`

pages/common/less-than.md

@@ -30,3 +30,7 @@
 - Pass command output to a program as a file descriptor:
 
 `diff <({{command1}}) <({{command2}})`
+
+- Open a persistent file descriptor:
+
+`exec {{3}}<{{path/to/file}}`