Migrate mqbcfg docs directory into a Doxygen file

This patch removes the old `doc/*.txt` file and migrates some
information in it into a file Doxygen can read.  The information not
migrated is:

  1. the table of contents for this package, which can be
     auto-generated from each component’s documentation, and

  2. the hierarchical synopsis.

This continues the changes made in commit 8c8af57.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

src/groups/mqb/mqbcfg/README.dox

@@ -0,0 +1,9 @@
+/**
+@dir mqbcfg
+
+@brief The `MQBCFG` (BlazingMQ Broker Config) package provide broker
+configuration schema and message component.
+
+This package provides the schema of the configuration used by the broker, as
+well as the generated corresponding messages component.
+*/