Add flex config for generic 433 MHz doorbell button

Amazon ASIN B01M1KB4UM (no brand/model name could be confirmed from
the listing). OOK PWM, 1000/3000/2000 us short/long/gap, matching the
working flex spec found by @zuckschwerdt in
https://github.com/merbanan/rtl_433/issues/1706. The device sends a
fixed, non-varying code with no visible id/button field to decompose
(only one button was ever captured), so this matches on the exact
code rather than extracting a field, following the same approach as
conf/friedlandevo.conf.

Verified against the real capture in the companion
rtl_433_tests#1706 fixture (17 repeats decoded), and confirmed it does
NOT match a second, differently-timed doorbell posted later in the
same issue thread.

conf/generic-doorbell-1000-3000.conf

@@ -0,0 +1,28 @@
+# Generic 433 MHz wireless doorbell button
+#
+# Amazon ASIN B01M1KB4UM; no brand/model name could be confirmed from the
+# listing. Sends a fixed, non-varying code with no visible id/button field
+# to decompose (only one button was ever captured), so this matches on the
+# exact code rather than extracting a field.
+#
+# The device uses OOK modulation with Pulse Width Coding (PWM) on 433.92 MHz:
+# - sync bit: ~4000 us
+# - long pulse: ~3000 us, short pulse: ~1000 us, fixed gap: ~2000 us
+#
+# Repeated code observed: {18}689b0 (16-bit match prefix used below since a
+# few repeats demodulate 1-2 bits short/long at the edges).
+#
+# Analyzed by @zuckschwerdt and @Ocean-85 in
+# https://github.com/merbanan/rtl_433/issues/1706
+
+decoder {
+    name        = Doorbell-Generic-B01M1KB4UM,
+    modulation  = OOK_PWM,
+    short       = 1000,
+    long        = 3000,
+    gap         = 2000,
+    reset       = 10000,
+    match       = 0x689b,
+    bits       >= 16,
+    unique,
+}