doc: scripts: gen_kconfig_rest: add workaround for page width

Since Zephyr docs switched to fixed-width, the list of Kconfig options
is not displayed correctly. We should move to another data
representation format, e.g. definition lists. Until this decision is
made, add a workaround that forces the template to expand page width to
the maximum on Kconfig list pages.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>