Remove redundant adjective in introduction.rst

The adjective "main" before "raison d'etre" in the second bullet of the last section (Organization of the documentation) is redundant. 

"raison d'etre" means "the most important reason for something's existence", so to say "the main raison d'etre" is to say "the main most important reason..."

I know this is pedantic, but I wanted to start contributing so I figured I'd tackle something easy the first time.

about/introduction.rst

@@ -83,7 +83,7 @@ relatively intuitive:
 - The :ref:`sec-general` section contains this introduction as well as
   information about the engine, its history, its licensing, authors, etc. It
   also contains the :ref:`doc_faq`.
-- The :ref:`sec-learn` section is the main *raison d'être* of this
+- The :ref:`sec-learn` section is the *raison d'être* of this
   documentation, as it contains all the necessary information on using the
   engine to make games. It starts with the :ref:`Step by step
   <toc-learn-step_by_step>` tutorial which should be the entry point for all