docs: netmem: document netmem and memory provider design principles

Add a Design Principles section to Documentation/networking/netmem.rst
covering the netmem_ref abstraction, the prohibition on direct
downcasting in callers, decoupling memory providers from net_iov,
decoupling net_iov from unreadability, delegating provider/type logic to
memory_provider_ops and netmem helpers, and the rule against mixing skb
fragment memory sources.

Cc: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20261010033630.1171692-3-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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