doc: genrest: Support customizing the description on index pages
doc: genrest: Support customizing the description on index pages

Extend the specifications passed to --modules to add the form

    <title>:<suffix>:<path>:<index description filename>

<index description filename> points to a file that contains RST that is
inserted at the top of the index page.

If no filename is passed, the old default description is used.

Also add three flags --top-index-desc, --non-module-index-desc, and
--all-index-desc for customizing the text at the top of non-module index
pages.

This functionality is currently unused in Zephyr, but will probably be
used later. It's being added for a downstream project.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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