Does anyone know a way to access the vertices and indices buffer from a compute shader?

The only way to get data into a compute shader is through a UAV, an SRV, or a constant buffer - so your input to the shader has to be one of those three options. Constant buffers are probably not a good choice, since their data can't change (hence the constant name :) ). SRVs are probably also not a great choice, since you will need to eventually store the data back to a buffer after you have sorted it. So most likely you would want to use a UAV to access the data, sort it, and then store it back to the UAV resource. Another option is to read the data from the SRV and write it to a UAV, which may help you with managing the data in flight.

Be aware that there are restrictions about which bind and usage flags a vertex buffer can have (test them out yourself - try creating a buffer resource with multiple combinations of bind flags, and also usage flags as well). This may force you to make an extra buffer that serves as a go-between for your algorithm passes.

RWStructuredBuffer<uint> is the resource representation in HLSL, and you use a UAV to bind the resource to the pipeline on the C++ side. So in that case, the UAV and the RWStructuredBuffer<uint> are representing the same resource, just in different domains.

CopyResource is a way to copy the contents of one resource to another. If you are starting out with an index buffer, and you want to copy it to another resource that has the ability to be bound as a UAV, then you would use copy resource to do that prior to invoking your compute shader.

The RWStructuredBuffer is actually the source of your data within the compute shader, and you will also need to write the sorted data back into it. That's where the name RW comes from - it is for reading and writing.

It sounds to me like you are unfamiliar with resources in D3D11 - I would recommend spending a bit of time with a good book to get familiar. Otherwise it will be a long hard struggle to get something trivially working... Any time that you invest in learning about resources is time well spent, so go for it!