docs: add link to the Zephyr new board checklist
Add a link to the index for the Zephyr new board checklist.
BRANCH=none
TEST=verify link in Gerrit browse
## Zephyr
+* [Zephyr New Board Checklist](./zephyr/zephyr_new_board_checklist.md)
* [Building Zephyr OS](./zephyr/zephyr_build.md)
* [Initialization Order](./zephyr/zephyr_init.md)
* [Proof-of-Concept-Device Bringup](./zephyr/zephyr_poc_device_bringup.md)
