Remove stale website pages

[jbednar]

The HoloViews web server provides some confusingly out of date pages like https://holoviews.org/user_guide/Styling_Plots.html that have changed names or been reorganized into pages like https://holoviews.org/user_guide/Style_Mapping.html . People clicking around at holoviews.org probably can't get to that material, but it still shows up in Google and appears to be curreyt, yet it has ancient advice like using .options. We might be keeping that old material around so that links stay working, but to me it seems like it should be a redirect to a current page to avoid giving people bad advice.