Added basic hex_lattice documentation
Added basic hex_lattice documentation

walshjon/openmc,amandalund/openmc,wbinventor/openmc,shikhar413/openmc,bhermanmit/openmc,mit-crpg/openmc,liangjg/openmc,lilulu/openmc,liangjg/openmc,shikhar413/openmc,amandalund/openmc,mit-crpg/openmc,liangjg/openmc,walshjon/openmc,lilulu/openmc,samuelshaner/openmc,mit-crpg/openmc,kellyrowland/openmc,shikhar413/openmc,wbinventor/openmc,paulromano/openmc,paulromano/openmc,paulromano/openmc,smharper/openmc,smharper/openmc,wbinventor/openmc,shikhar413/openmc,mjlong/openmc,johnnyliu27/openmc,samuelshaner/openmc,mit-crpg/openmc,walshjon/openmc,paulromano/openmc,smharper/openmc,kellyrowland/openmc,amandalund/openmc,samuelshaner/openmc,johnnyliu27/openmc,samuelshaner/openmc,walshjon/openmc,smharper/openmc,wbinventor/openmc,mjlong/openmc,lilulu/openmc,amandalund/openmc,bhermanmit/openmc,johnnyliu27/openmc,johnnyliu27/openmc,liangjg/openmc