Introduce doc/examples/ directory

It contains simple usual setups to get started with osmo-gsm-tester,
such as creating a 4G network using srsLTE or a 2G network using the
osmocom stack (+ ofono modems).

Change-Id: I55d5b1152da201799af21a77ef6c562f97bd94fc
diff --git a/doc/examples/4g_srsLTE/scenarios/mod-enb-nprb@.conf b/doc/examples/4g_srsLTE/scenarios/mod-enb-nprb@.conf
new file mode 100644
index 0000000..f17347c
--- /dev/null
+++ b/doc/examples/4g_srsLTE/scenarios/mod-enb-nprb@.conf
@@ -0,0 +1,3 @@
+modifiers:
+  enb:
+  - num_prb: ${param1}