What

Update the POST /pages/<page_index>/proof RPC to:

index it by slot_id,
change its method to GET, and
change its path to /levels/<level>/slots/<slot_index>/pages/<page_index>/proof

Why

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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