doc: board: toradex: add colibri_imx7.rst

- add initial index for toradex boards reST documentation
- add initial colibri_imx7.rst doc file which provides all needed
information for obtaining a workable image ready for flashing
for both eMMC/NAND versions of Colibri iMX7.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: make title underline the same length as the title itself]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>