zephyr: doc: Add zephyr docs to sitemap
To make it easier for people to get started with Zephyr, link in the
existing docs.
BRANCH=none
TEST=check it in gitiles
* [EC-3PO overview](./ec-3po.md)
* [EC-3PO design doc](./ec-3po-design.md)
+## Zephyr
+
+* [Initialization Order](./zephyr_init.md)
+* [Proof-of-Concept-Device Bringup](./zephyr_poc_device_bringup.md)
+* [Shimming](./zephyr_shim.md)
+* [Porting EC unit tests to Ztest](./ztest.md)
+
## Miscellaneous
* [Low Battery Startup](./low_battery_startup.md)
