Documentation: networking: dsa: demote subsections to simple emphasized words

"make htmldocs" complains:
configuration.rst:165: WARNING: duplicate label networking/dsa/configuration:single port, other instance in (...)
configuration.rst:212: WARNING: duplicate label networking/dsa/configuration:bridge, other instance in (...)
configuration.rst:252: WARNING: duplicate label networking/dsa/configuration:gateway, other instance in (...)

And for good reason, because the "single port", "bridge" and "gateway"
use cases are replicated twice, once for normal taggers and twice for
DSA_TAG_PROTO_NONE. So when trying to reference these sections via a
hyperlink such as:

https://www.kernel.org/doc/html/latest/networking/dsa/configuration.html#single-port



it will always reference the first occurrence, and never the second one.

This change makes the "single port", "bridge" and "gateway"
configuration examples consistent with the formatting used in the
"Configuration showcases" subsection.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>