Move links to Cheatsheet and Glossary into the Reference submenu

Also tweak the wording of the API entry in the Scala 3 landing page.

Author: julienrf

_data/doc-nav-header.yml

@@ -31,20 +31,20 @@
       url: "/books.html"
     - title: Scala FAQ
       url: "/tutorials/FAQ/index.html"
-    - title: Language Spec
+    - title: Language Specification
       url: http://scala-lang.org/files/archive/spec/2.13/
     - title: Style Guide
       url: "/style/index.html"
+    - title: Cheatsheet
+      url: "/cheatsheets/index.html"
+    - title: Glossary
+      url: "/glossary/index.html"
 - title: API
   url: "#"
   submenu:
     - title: Current
       url: https://www.scala-lang.org/api/current/
     - title: All Versions
       url: "/api/all.html"
-- title: Cheatsheet
-  url: "/cheatsheets/index.html"
-- title: Glossary
-  url: "/glossary/index.html"
 - title: SIPs
   url: "/sips/index.html"

index.md

@@ -32,8 +32,8 @@ scala3-sections:
         description: "Detailed guides about particular aspects of the language."
         icon: "fa fa-map"
         link: /scala3/guides.html
-      - title: "API"
-        description: "API documentation for every version of Scala 3."
+      - title: "Scala Library API"
+        description: "API documentation for every version of the Scala 3 standard library."
         icon: "fa fa-file-alt"
         link: https://scala-lang.org/api/3.x/
       - title: "Language Reference"