Initial minimal OsmoHNBGW user manual

This adds a very basic OsmoHNBGW user manual with an "overview" and
"running" chapter.  It still misses any information on configuration.

Change-Id: I54825a4ce5a457f87b0618332347eed8ea54afcb
Closes: OS#2588
diff --git a/.gitignore b/.gitignore
index f662e2a..49b2cfb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,3 +43,8 @@
 include/osmocom/rua/rua_ies_defs.h
 include/osmocom/ranap/ranap_ies_defs.h
 include/osmocom/sabp/sabp_ies_defs.h
+
+doc/manuals/build
+doc/manuals/common
+doc/manuals/generated
+doc/manuals/*.pdf