zephyr: docs: add PD documentation
zephyr: docs: add PD documentation

Document the PD Kconfig.pd options and
sub-options

BUG=b:208437225
TEST=Browse doc in gerrit
BRANCH=main

Signed-off-by: Sam Hurst <500b4cddcd66abe3fe333e92c4c95d7f41e62e4e@google.com>
Change-Id: I2a6e1e193637bc5089b6f77e135745e27c46c7b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3451256
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec