Add page id feature

WebsiteGenerator/Templates/Filled/LocalizedSiteTemplate.swift

@@ -58,7 +58,7 @@ struct LocalizedSiteTemplate {
         let sections = site.sortedItems.map {
             PrefilledTopBarTemplate.SectionInfo(
                 name: $0.title(for: language),
-                url: "\($0.path)/\(language).html")
+                url: $0.path + Element.htmlPagePathAddition(for: language))
         }
 
         self.topBar = try .init(
@@ -83,7 +83,7 @@ struct LocalizedSiteTemplate {
     func makeBackLink(text: String, language: String) -> String {
         let content: [BackNavigationTemplate.Key : String] = [
             .text: text,
-            .url: "../\(language).html"
+            .url: ".." + Element.htmlPagePathAddition(for: language)
         ]
         return backNavigation.generate(content)
     }