Immutable Resources fix

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Objective

Fixes #24285.

Solution

Add UnsafeWorldCell::get_resource_assume_mutable, World::modify_resource and World::modify_resource_by_id and updated the migration guide.

Testing

The new methods are essentially identical to their component counterparts.

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I don't know where all the migration guides have gone to, but for future reference: I'd like to add the following to the resources_as_components migration guide:

If it's not possible to add the bound, use either `UnsafeWorldCell::get_resource_mut_assume_mutable`, `UnsafeWorldCell::get_resource_by_id` and make sure that the safety conditions are satisfied. 
There's also `World::modify_resource` and `World::modify_resource_by_id`, that behave exactly like their component counterparts.
Lastly, it's also possible to use the `*_assume_mutable` component methods for this purpose, but you'd first have to retrieve the resource entity from `ResourceEntities`.

And also the PR number on the top.