[doc] fix documentation job by using numpy 1.26 instead of stable in sphinx

Documentation jobs are failing on warnings in pipelines since 1 day. It seems to be related to the release of numpy 2.0:

Examples:

• https://gitlab.idiap.ch/biosignal/software/mednet/-/jobs/405268
• https://gitlab.idiap.ch/biosignal/software/mednet/-/jobs/406034

Error:

:1:py:class reference target not found: numpy.bool_

Reproducing:

On main branch: pixi run -e default doc

Proposed fix:

Modifing auto_intersphinx_packages to use explicitly the doc for version 1.26 instead of stable (which is now 2.0)