doc: Re-order top-level documents to improve usability
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 <1038bc33971d2c23c8e2eee8dd6938c077db52dc@windriver.com>

mirzak/zephyr-os,finikorg/zephyr,nashif/zephyr,runchip/zephyr-cc3200,erwango/zephyr,holtmann/zephyr,mirzak/zephyr-os,bboozzoo/zephyr,sharronliu/zephyr,runchip/zephyr-cc3220,runchip/zephyr-cc3200,sharronliu/zephyr,explora26/zephyr,rsalveti/zephyr,rsalveti/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,holtmann/zephyr,kraj/zephyr,galak/zephyr,bigdinotech/zephyr,tidyjiang8/zephyr-doc,erwango/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,tidyjiang8/zephyr-doc,sharronliu/zephyr,punitvara/zephyr,finikorg/zephyr,fractalclone/zephyr-riscv,tidyjiang8/zephyr-doc,bboozzoo/zephyr,holtmann/zephyr,galak/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,mbolivar/zephyr,pklazy/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,mirzak/zephyr-os,zephyriot/zephyr,fbsder/zephyr,mirzak/zephyr-os,runchip/zephyr-cc3220,runchip/zephyr-cc3220,rsalveti/zephyr,sharronliu/zephyr,bigdinotech/zephyr,mbolivar/zephyr,fractalclone/zephyr-riscv,zephyriot/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,holtmann/zephyr,nashif/zephyr,erwango/zephyr,finikorg/zephyr,mirzak/zephyr-os,bboozzoo/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,pklazy/zephyr,explora26/zephyr,zephyriot/zephyr,kraj/zephyr,mbolivar/zephyr,Vudentz/zephyr,explora26/zephyr,Vudentz/zephyr,fractalclone/zephyr-riscv,bigdinotech/zephyr,tidyjiang8/zephyr-doc,ldts/zephyr,punitvara/zephyr,finikorg/zephyr,Vudentz/zephyr,erwango/zephyr,fbsder/zephyr,pklazy/zephyr,pklazy/zephyr,nashif/zephyr,bigdinotech/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,ldts/zephyr,erwango/zephyr,kraj/zephyr,galak/zephyr,runchip/zephyr-cc3200,holtmann/zephyr,Vudentz/zephyr,rsalveti/zephyr,punitvara/zephyr,galak/zephyr,bboozzoo/zephyr,pklazy/zephyr,sharronliu/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,runchip/zephyr-cc3200,explora26/zephyr,fbsder/zephyr,mbolivar/zephyr,bigdinotech/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,fractalclone/zephyr-riscv,ldts/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,nashif/zephyr,rsalveti/zephyr,bboozzoo/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr