docs: networking: add full DIM API

Add the full net DIM API to the net_dim.rst file.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

docs: networking: convert DIM to RST

Convert the Dynamic Interrupt Moderation doc to RST and
use the RST features like syntax highlight, function and
structure documentation, enumerations, table of contents.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>