doc: tweak CSS for option table display

Noticed in docs.zephyrproject.org/subsystems/test/sanitycheck.html that
the first column of the option table display is too narrow (for short
option names that are shown on the same line as the description).  This
PR tweaks the CSS to widen the first column enough to stop wrapping in
most cases.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>