Make a few memory management objects public + Miscellaneous doc updates

[leofang]

Description

closes #596.

• Buffer and MemoryResource are already public (ex: returned by Device.memory_resource.allocate()). We just expose them under the cuda.core namespace for documentation purposes (Doc: Document cuda.core public (but non-entry-point) objects #601)
  • I noticed that Buffer's destructor would use the default stream if stream is not passed explicitly. This isn't right because it takes away the control from the memory resource authors. We now always let the memory resource make the call.
  • I noticed Buffer has an __init__(). This isn't right (since we want buffers to be returned by MR.allocate()) if we want to make it public. I moved this support to from_handle().
• The new exposed objects DeviceMemoryResource and LegacyPinnedMemoryResource are named after their respective cccl-rt/cudax counterparts.
• The __cuda_stream__ protocol has a protocol type now (IsStreamT)
• Internal refactoring to consolidate support for objects that have __cuda_stream__
• Documentation is clarified and expanded to reflect the new public exposure of these classes, in particular many type hints are updated/fixed
• Update the docs to mention all other merged PRs targeting this release (v0.3.0)

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[leofang]

/ok to test a6387a0

[github-actions]

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[leofang]

/ok to test cc0f6ce

[leofang]

/ok to test 58323ac