Got these warning while building docs. RW2006 is not linked to the ref book URL.

\doc\modules\gaussian_process.rst:68: WARNING: citation not found: RW2006
\doc\modules\gaussian_process.rst:167: WARNING: citation not found: RW2006
\doc\modules\gaussian_process.rst:190: WARNING: citation not found: RW2006
\doc\modules\gaussian_process.rst:246: WARNING: citation not found: RW2006
\doc\modules\gaussian_process.rst:356: WARNING: citation not found: RW2006
\doc\modules\gaussian_process.rst:541: WARNING: citation not found: RW2006