docs: associate flutter with dart to improve discoverability (#1022)

Author: ghuntley

src/docs/languages-and-frameworks.md

@@ -22,4 +22,5 @@ Below is a list of language and framework specific tips & tricks.
 - [Kotlin](/docs/languages/kotlin)
 - [Pandas](/docs/languages/python/#pandas)
 - [Deno](/docs/languages/deno)
+- [Flutter](/docs/languages/dart#flutter)
 - [Elm](/docs/languages/elm)

src/docs/menu.ts

@@ -228,6 +228,10 @@ export const MENU: MenuEntry[] = [
                 "Dart",
                 "languages/dart"
             ),
+            M(
+                "Flutter",
+                "languages/dart#flutter"
+            ),
             M(
                 "Julia",
                 "languages/julia"