Clarify Contacts docstrings for rigid contact points and offsets

[preist-nvidia]

The newton.Contacts docstrings for rigid_contact_point0/1 and rigid_contact_offset0/1 are currently too vague.

Please make these docstrings more specific about what each quantity represents and state clearly which frame they are stored in, so the generated API docs are not ambiguous. The current wording makes it hard to understand the distinction between the contact points and the contact offsets.