Tutorials in docs

[Vaibhavdixit02]

The docs don't have any tutorials, for a start we should just add the test there and iterate from there.

[ChrisRackauckas]

Yeah this is a pretty big one. It should have an example doing something with an ODE with global optimizers, being used with Flux with Zygote, Newton methods on something else, etc.

[mkg33]

I'm currently working on adding more content. For now, I'm just adding the tests and moving introduction to the 'basics' section.