minor #18186 Revamping Multiple Kernels documentation (yceruto)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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Revamping Multiple Kernels documentation

As we discussed on Slack, this approach will simplify and remove the boilerplate code while the result and goals are the same as before.

Commits
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f1698a4 Revamping Multiple Kernels documentation

Author: javiereguiluz