doc: fix matrix CSS for recent sphinx
doc: fix matrix CSS for recent sphinx

It seems sphinx >= 2.0 is inserting a <p> tag in each table cell.
The feature table (matrix) style needs to be updated to avoid
cells being too big.

The margin, padding and line height are overridden.
The font size in percentage is replaced with an equivalent pixel size.
The border is explicit because it disappeared for th.

Signed-off-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk