[doc] Update documenation section names

This commit updates various section names to make the auto generated
menus and siteContent lists more intuitive.

After this change the top level menu will look as follows:

OpenTitan
  - Documentation
  - Hardware
  - Software
  - Utilities

It previously was:

Introduction to OpenTitan
  - Documentation Index
  - Hardware Dashboard
  - OpenTitan Software
  - OpenTitan Utilities

Signed-off-by: Miguel Osorio <miguelosorio@google.com>
diff --git a/doc/_index.md b/doc/_index.md
index 277ea46..daab8da 100644
--- a/doc/_index.md
+++ b/doc/_index.md
@@ -1,5 +1,5 @@
 ---
-title: "Documentation Index"
+title: "Documentation"
 ---
 
 {{% sectionContent %}}