Use file name in page identifiers
Use file name in page identifiers

Partially reverts an earlier change to move away from using file names
directly (by dropping the extension). However, navigation links require
full filenames, so it makes sense to keep the full file name in the
`doc.internal.pages` vector.

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