Fix developer guide not compiling

This MR fixes a compile error in the developer-guide documentation:

(/usr/share/texlive/texmf-dist/tex/luatex/interpreter/interpreter.tex
! Undefined control sequence.
l.8 \directlua
              {require("interpreter.lua")}

This arises due to pdflatex picking up the luatex interpreter file instead of the Nektar++ interpreter documentation.

Nektar++ file has been renamed to avoid this.