doc: fix broken text search
doc: fix broken text search

Require Sphinx<3.4.0 to avoid the issue described in:

https://github.com/sphinx-doc/sphinx/issues/8603

This requirement can be relaxed once a new sphinx-rtd-theme is
released.

Signed-off-by: Fabio Utzig <382671175562e0df8128054dbdb05a250d79f588@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr