doc-rst: Reduce CSS padding around Field

doc-rst: Reduce CSS padding around Field

Right now any ":Field Name: Field Contents" lines end up with significant
padding due to CSS from the "table" CSS which rightly needs padding to
make tables readable. However, field lists don't need this as they tend
to be stacked together. The future heavy use of fields in the parsed
MAINTAINERS file needs this cleaned up, and existing users look better
too. Note the needless white space (and misalignment of name/contents)
between "Date" and "Author":

https://www.kernel.org/doc/html/latest/accounting/psi.html

This patch fixes this by lowering the padding with a more specific CSS.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>