gallery toctree path should not use os separator, always backslash (#6349)
gallery toctree path should not use os separator, always backslash (#6349)

* gallery toctree path should not use os separator, always backslash

* added comment and config option to set custom gallery dir

* updated comment

DuCorey/bokeh,philippjfr/bokeh,timsnyder/bokeh,dennisobrien/bokeh,bokeh/bokeh,DuCorey/bokeh,bokeh/bokeh,Karel-van-de-Plassche/bokeh,philippjfr/bokeh,Karel-van-de-Plassche/bokeh,aavanian/bokeh,jakirkham/bokeh,ericmjl/bokeh,rs2/bokeh,dennisobrien/bokeh,jakirkham/bokeh,dennisobrien/bokeh,bokeh/bokeh,timsnyder/bokeh,Karel-van-de-Plassche/bokeh,mindriot101/bokeh,stonebig/bokeh,jakirkham/bokeh,aavanian/bokeh,rs2/bokeh,ericmjl/bokeh,mindriot101/bokeh,ericmjl/bokeh,philippjfr/bokeh,jakirkham/bokeh,aavanian/bokeh,timsnyder/bokeh,dennisobrien/bokeh,aavanian/bokeh,ericmjl/bokeh,bokeh/bokeh,DuCorey/bokeh,rs2/bokeh,philippjfr/bokeh,Karel-van-de-Plassche/bokeh,mindriot101/bokeh,aavanian/bokeh,DuCorey/bokeh,timsnyder/bokeh,mindriot101/bokeh,jakirkham/bokeh,stonebig/bokeh,timsnyder/bokeh,bokeh/bokeh,stonebig/bokeh,DuCorey/bokeh,rs2/bokeh,stonebig/bokeh,ericmjl/bokeh,Karel-van-de-Plassche/bokeh,dennisobrien/bokeh,rs2/bokeh,philippjfr/bokeh