docs: add moderation and viewer-state guides to www, bump version to v0.20.0

www/src/www/config.gleam

···
         #("guides/queries.md", "/guides/queries", "Queries"),
         #("guides/joins.md", "/guides/joins", "Joins"),
         #("guides/mutations.md", "/guides/mutations", "Mutations"),
+        #("guides/moderation.md", "/guides/moderation", "Moderation"),
+        #("guides/viewer-state.md", "/guides/viewer-state", "Viewer State"),
         #(
           "guides/authentication.md",
           "/guides/authentication",

www/src/www/layout.gleam

···
   ])
 }
 
-const version = "v0.17.5"
+const version = "v0.20.0"
 
 fn sidebar(current_path: String, pages: List(DocPage)) -> Element(Nil) {
   html.aside([attribute.class("sidebar")], [