Limitations documentation and more descriptive implementation error messages

Currently, `dkpy` does not provide much information for `NotImplementedError` messages for features such as real, diagonal, and non-square uncertainty, as it mentioned in this [comment](https://github.com/decargroup/dkpy/issues/39#issuecomment-3002138177) in #39.

Therefore, it would be better to provide the information that these features are not implemented library-wide (i.e., independent of the D-scale fit, controller synthesis method, etc.) and that they are currently under development. Moreover, it may be important to highlight these current limitations of the library in the documentation as well as providing some standard workarounds for the time being.