only deploy to stable if we build the latest tag (#708)
only deploy to stable if we build the latest tag (#708)

* only deploy to stable if we build the latest tag

* don't deploy pre-release docs to stable

* make sure tag is a valid VersionNumber

mortenpi/Documenter.jl,JuliaDocs/Documenter.jl,MichaelHatherly/Lapidary.jl