Docs refresh (work-in-progress)

We should probably add some text here as follows: 

The provided OPDs are based on the observatory design requirements, and were developed for the Mission Critical Design Review. The represent the nominal case of performance for JWST, and have not yet been updated with as-built details of mirror surface figures, etc.   We intend to make updated OPD files available once suitable reference data have been provided to STScI.   For now, see `Lightsey et al. 2014 <http://adsabs.harvard.edu/abs/2014SPIE.9143E..04L>`_ for recent predictions of JWST's likely performance.