Initial OsmoGbPROXY user manual

This adds a very basic manual consisting of nothing more than
the common chapters and a high-level description of what it is
all about.

Change-Id: I80d4ea016376c59995ccfcd8685c7c0e86745bd2
diff --git a/doc/manuals/chapters/gbproxy-configuration.adoc b/doc/manuals/chapters/gbproxy-configuration.adoc
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+== Configuring OsmoGbPROXY
+
+TBD.  Unfortunately this chapter of the manual still needs to be written.
+Osmocom has very limited funding and support resources; Feel free to help
+us completing this documentation by contributing with code, documentation
+or by supporting the developers financially.