Add warning to set zmin and zmax for ion-acc example

[n01r]

This PR adds a warning to the documentation of the laser-ion acceleration example to set <species>.zmin / zmax / xmin / ... for simulations of highly dense, spatially confined/limited targets to avoid particle creation throughout the whole box which could lead to memory overflows on the device.

It also changes the ion acceleration example use these parameters.

[ax3l]

@n01r looks like this changed some of the target params sufficiently in the example? either way, please plot quickly if the target is as intended, paste the figure here and then reset the checksum please :)

[ax3l]

@n01r can you please update the checksum?

[ax3l]

@n01r gentle reminder to update the PR :)

[n01r]

Sorry about forgetting about this for so long, @ax3l. Which checksum needs to be reset?

The densities are exactly the same (just showing electron density here). I had to double-check that the inputs of my two test simulations differed.

[n01r]

@ax3l, I now reset the checksum according to our docs.

[ax3l]

Rebased to fix macOS CI.