doc: new 'mptcp' page in 'networking'

A general documentation about MPTCP was missing since its introduction
in v5.6.

Most of what is there comes from our recently updated mptcp.dev website,
with additional links to resources from the kernel documentation.

This is a first version, mainly targeting app developers and users.

Link: https://www.mptcp.dev


Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240530-upstream-net-20240520-mptcp-doc-v3-3-e94cdd9f2673@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>