Avoid circular dependencies for docs build

[sethaxen]

Currently our docs build depends on ArviZPythonPlots, which itself depends on ArviZ. As a result, when we make a breaking change here or bump the compat of a dependency that ArviZPythonPlots also depends on, we can't build the docs in CI or test APP with that dependency in CI. Instead, we need to update both compats locally andrun the tests locally. This is not ideal for development.

Our docs build only depends on ArviZPythonPlots for tutorials. Perhaps we should a similar approach as turinglang.org. Since Turing is a metapackage, they only include package docs for the component packages. The docs for the tutorials are hosted at https://github.com/TuringLang/docs and built to https://turinglang.org/docs, while the main docs are hosted at https://github.com/TuringLang/turinglang.github.io and built to https://turinglang.org/