doc(demo): add another example into demo set

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

doc/demo/congr.lean

@@ -0,0 +1,25 @@
+rewrite_set simple
+add_rewrite and_truer and_truel and_falser and_falsel or_falsel : simple
+(*
+  add_congr_theorem("simple", "and_congr")
+  add_congr_theorem("simple", "or_congr")
+*)
+
+variables a b c : Nat
+
+(*
+local t = parse_lean([[a = 1 ∧ (¬ b = 0 ∨ c ≠ 0 ∨ b + c > a)]])
+local s, pr = simplify(t, "simple")
+print(s)
+print(pr)
+print(get_environment():type_check(pr))
+*)
+
+exit
+
+check @and_congr
+check @or_congr
+check @and_congrr
+check @and_congrl
+import if_then_else
+check @if_congr