docs: simplify api docs generation for comparison

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• PR created as a comparative reference for discussion in docs: classes generated docs sometimes include non-traits members, and sometimes do #4252

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/spawner.html#spawner
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/spawner.html#spawner

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/spawner.html#localprocessspawner
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/spawner.html#localprocessspawner

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/user.html#user
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/user.html#user

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/service.html#service
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/service.html#service

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/proxy.html#configurablehttpproxy
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/proxy.html#configurablehttpproxy

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This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/auth.html#dummyauthenticator
Main branch: https://jupyterhub.readthedocs.io/en/latest/api/auth.html#dummyauthenticator

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• This PR: https://jupyterhub--4253.org.readthedocs.build/en/4253/api/auth.html#pamauthenticator
• Main branch: https://jupyterhub.readthedocs.io/en/latest/api/auth.html#pamauthenticator

[minrk]

Nice! I think we probably should use :members: most places. Some members that should be documented are likely inherited, which need to be explicitly included or use :inherited-members:.

Part of this is that I haven't been careful about prefixing private APIs with _, which I should have, because extension endpoints like Spawners have a bit more access to objects than I realized. We should go through the User and Spawner APIs and migrate some things to private APIs, deprecating some public-looking names.

Service has no API because no extension APIs have handles on them, so those API docs are purely for jupyterhub contributor reference.

[minrk]

After thinking a bit about this, I think we mostly don't want :members: on most of these, as the docs are meant to cover configuring these classes, not their Python APIs, which are only used for developing JupyterHub or custom Spawners/Authenticators.

I think User is probably the class where I've done the most "accidental public API" by exposing attributes that shouldn't be public without a _private prefix.

I think maybe even Spawner and Authenticator should occur in two separate places: one config-only for deployers, and one with public API methods for developers. Because implementation details can be very distracting for someone looking for a configuration reference. WDYT?