Add tutorial_write_in_kernel to the docs page

[erikvansebille]

Description

While #2620 implemented the functionality to write particle sets to output inside a Kernel and provided a tutorial for it, I now realise that that tutorial was not added to the docs page, so was not easily findable there. This PR adds it

Checklist

• Closes Explore and expose use of writing particles in a Kernel #2599
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

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[erikvansebille]

Oops, I now see that I had already addressed this in #2657, so closing here