doc: Re-order top-level documents to improve usability

Groups top-level documents so that related documents are adjacent.
Also re-orders things so that documents for newcomers appear before
documents on more advanced topics.

The rationale behind the resulting order is described below:

Getting Started Guide
- Enables a newcomer to access & try out Zephyr by running a sample
  application using QEMU.

Supported Boards
- Enables a newcomer to try out Zephyr by running a sample application
  using actual hardware.
- Also lets newcomers know which boards are currently supported.

Zephyr Kernel Primer
Device Drivers and Device Model
Subsystems
- Describes the various components of the Zephyr kernel.
- Lets a developer know what is available when they are *designing* code.

API Documentation
- Describes the various APIs of the Zephyr kernel.
- Lets a developer know what is available when they are *writing* code.

Application Development Primer
- Provides the info needed for a developer to create a new application
  using Zephyr.

Contributing Code
- Tells a developer how to contribute code back to the Zephyr project.
- Advanced topic.

Porting Guides
- Provides guidelines for porting Zephyr to a new architecture.
- Advanced topic.

Build System User Guide
- Covers writing Kconfig options, and also contains reference guide.
- Advanced topic.

Change-Id: I8ecb4953c788e843028d1e8778be1ef1aee6c175
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>

doc/index.rst

@@ -29,15 +29,15 @@ Sections
    :maxdepth: 1
 
    getting_started/getting_started.rst
+   board/board.rst
    kernel/kernel.rst
-   contribute/code.rst
    drivers/drivers.rst
    subsystems/subsystems.rst
-   porting/porting.rst
    api/api.rst
-   board/board.rst
-   reference/kbuild/kbuild.rst
    application/application.rst
+   contribute/code.rst
+   porting/porting.rst
+   reference/kbuild/kbuild.rst
 
 You can find further information on the `Zephyr Project Wiki`_.