Improve documentation of the numpy_usm_shared module

[diptorupd]

We should add

• a module-level doctring with examples.
• class level docstrings in ndarray with examples.
• from_ndarray and as_ndarray functions
• include the docs into dpctl sphinx docs

[PokhodenkoSA]

@diptorupd Is it possible to make it for the current release?

[diptorupd]

It will still be good to do, but I do not see it happening. numpy_usm_shared is in maintenance mode and will be deprecated once dpctl.tensor.usm_ndarray is feature complete.
As such closing as WONTFIX