osmotrx: Create a common chapter for section documenting backends

Change-Id: I6bea1ccca4ce72b92641a36f9f5894ac9a6cae72
diff --git a/doc/manuals/chapters/trx-backends.adoc b/doc/manuals/chapters/trx-backends.adoc
index e801886..8829fa6 100644
--- a/doc/manuals/chapters/trx-backends.adoc
+++ b/doc/manuals/chapters/trx-backends.adoc
@@ -1,5 +1,8 @@
+[[trx_backends]]
+== OsmoTRX backend support
+
 [[backend_uhd]]
-== `osmo-trx-uhd` for UHD based Transceivers
+=== `osmo-trx-uhd` for UHD based Transceivers
 
 This OsmoTRX model uses _libuhd_ (UHD, USRP Hardware Driver) to drive the
 device, that is configuring it and reading/writing samples from/to it.
@@ -17,7 +20,7 @@
 _osmo-trx.git_.
 
 [[backend_lms]]
-== `osmo-trx-lms` for LimeSuite based Transceivers
+=== `osmo-trx-lms` for LimeSuite based Transceivers
 
 This OsmoTRX model uses LimeSuite API and library to drive the device, that is
 configuring it and reading/writing samples from/to it.
@@ -30,7 +33,7 @@
 _osmo-trx.git_.
 
 [[backend_usrp1]]
-== `osmo-trx-usrp1` for libusrp based Transceivers
+=== `osmo-trx-usrp1` for libusrp based Transceivers
 
 This OsmoTRX model uses the legacy libusrp driver provided in GNU Radio 3.4.2.