docs: SysVI docs

[ori-kron-wis]

@Hrovatin feel free to add a model description to /Users/orikr/PycharmProjects/scvi-tools/docs/user_guide/models/sysvi.md, in this PR

[codecov]

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[Hrovatin]

@ori-kron-wis thank you very much.
I see many of the other models have small tutorials in the md file and a lot of additional theory behind it. What would be your general guide on how much info we should add here, especially since we have a detailed tutorial in notebook.

[ori-kron-wis]

@Hrovatin , well, it's up to you.
While the tutorial aim is to show an end-to-end typical use case of your model, the model description aim is to have a more theoretical background on it (see some other example, usually we expect to see advantages & limitations of model , the generative and inference processes scheme and tasks it can perform).
I can see your tutorial has some theoretical parts, it's fine, they can have some overlap between them.

As I said, it's up to you, we do have other external models with no theoretical background in RTD, which is a bit pitty, as it reduces the chances of users to really adopting the model.