docs: netdev: convert to a non-FAQ document

The netdev-FAQ document has grown over the years to the point
where finding information in it is somewhat challenging.
The length of the questions prevents readers from locating
content that's relevant at a glance.

Convert to a more standard documentation format with sections
and sub-sections rather than questions and answers.

The content edits are limited to what's necessary to change
the format, and very minor clarifications.
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>