doc: Move the b4 guide into sending_patches

The b4 contributor guide sits in the coding-style document, which is an
odd place for it. Move it into sending_patches.rst, next to the patman
note, so both patch-sending tools are described together.

The b4_contrib label moves with it, so the reference from process.rst
still resolves.

Signed-off-by: Simon Glass <[email protected]>
Reviewed-by: Mattijs Korpershoek <[email protected]>
Reviewed-by: Tom Rini <[email protected]>

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