doc: toolchains: Add Zephyr SDK documentation

This commit adds a dedicated page for the Zephyr SDK under the
'Toolchains' sub-category under 'Developing with Zephyr'.

The content of this page is based on the Zephyr SDK installation
instruction from the 'Getting Started Guide'.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>