Correct usage of computation_settings

[NithyasriVS]

This is PR proposed, inspired from #51

Using a configuration where all computation settings are set to False may not be a meaningful way to use qibotn as the user no longer knows what their output is a computation of, especially as more features/computations get added to the QuimbBackend in the future. So, it could be useful to restrict the user from wanting to use the configuration where are settings are False

Thus, a note has been added in the docs under "Setting the runcard" section as follows:

"Note that either MPS_enabled or expectation_enabled must be set to True to be able to perform any useful calulations with the QuimbBackend. Thus, it is compulsary to set one of them to true according to whether the user wants to calculate the dense vector or expectation value."

[NithyasriVS]

Discussion done on the same and concluded that documentation can be further extended if needed in the future.