Migrate mqbcmd 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, because component brief
     descriptions are already coptured in each component’s documentation and
     wil be automatically generated by Doxygen, and

  2. the hierarchical synopsis.

This continues the changes made in 69883e2.

Signed-off-by: Patrick M. Niedzielski <patrick@pniedzielski.net>

src/groups/mqb/mqbcmd/README.dox

@@ -0,0 +1,6 @@
+/**
+@dir mqbcmd
+
+@brief The `MQBCMD` (BlazingMQ Commands) package provides a parser and
+       manipulator of command types.
+*/