Overview docs for analysis nbsearch

Overview docs for analysis nbsearch

Add doxygen documentation that provides an overview of the analysis
neighborhood search, both from a usage perspective and from an
implementation perspective.  While the latter may not be that important
for using the API, it is probably of interest for people with general
background in MD.  And it shows the complexity of the implementation,
providing additional justification for having this as a shared
implementation for all the tools.

Change-Id: If52060b93601cbf051db6062ef644900dcb1f780