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 <Ulf.Magnusson@nordicsemi.no>