chore: remove accidentally committed generated file and gitignore it

chapter-listings.json is generated by the ListingManager tool and
should not be tracked in source control.

Author: BenjaminMichaelis

.gitignore

@@ -38,6 +38,7 @@ EssentialCSharp.Web/wwwroot/Chapters
 EssentialCSharp.Web/wwwroot/sitemap.xml
 EssentialCSharp.Web/Chapters/
 EssentialCSharp.Web/ListingSourceCode
+EssentialCSharp.Web/wwwroot/js/chapter-listings.json
 Utilities/EssentialCSharp.Web/Chapters/
 Utilities/EssentialCSharp.Web/wwwroot/sitemap.xml
 Utilities/EssentialCSharp.Web/wwwroot/Chapters/