doc: update v2.4 release notes

Update draft release notes with more information about documentation.
Remove code-block extra indenting.
Add label to roscube gsg so we can link to it (in the releaes notes).
Fix style for :option: references to make them look more links links.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/static/acrn-custom.css

@@ -30,6 +30,11 @@
    font-weight: bold;
 }
 
+/* :option: link color */
+a code span.pre {
+   color: #2980b9;
+}
+
 /* tweak doc version selection */
 .rst-versions {
    position: static;