doc: doxygen: adjust titles

Adjust the project title and the home page to make the sidebar tree more
usable.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/_doxygen/mainpage.md

@@ -1,6 +1,4 @@
-# API Documentation
-
-## Introduction
+# Introduction
 
 The Zephyr OS is a small-footprint kernel designed for use on
 resource-constrained and embedded systems: from simple embedded

doc/zephyr.doxyfile.in

@@ -32,7 +32,7 @@ DOXYFILE_ENCODING      = UTF-8
 # title of most generated pages and in a few other places.
 # The default value is: My Project.
 
-PROJECT_NAME           = "Zephyr Project"
+PROJECT_NAME           = "Zephyr API Documentation"
 
 # The PROJECT_NUMBER tag can be used to enter a project or revision number. This
 # could be handy for archiving the generated documentation or if some version