doc: Update coding style to recommend b4 for patch submission

Replace references to patman with b4 as the recommended tool for
preparing and sending patches. b4 is widely adopted in the Linux kernel
community and U-Boot ships with configuration to simplify its use with
the project mailing list.

The updated guidelines describe how to prepare series with b4, handle
cover letters and recipient lists, run style checks, and send patches
safely. Instructions also highlight how to collect and apply review tags
before resending.

This change documents the preferred workflow for contributors and
ensures consistency with common practices across related upstream
communities.

Reviewed-by: Neil Armstrong <[email protected]>
Reviewed-by: Mattijs Korpershoek <[email protected]>
Signed-off-by: Javier Tia <[email protected]>

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