[Tracker] Improve Tutorial

This issue is not labeled “contribution-welcome”, though, if you are interested in, don’t be shy to contact me 😃

What I’m going to do in the next months as follows:

1. Align the contents of tutorials with the README’s Key Features
2. Add https://pytorch.org/tutorials/beginner/deep_learning_60min_blitz.html -ish thing
3. Add more contents as “Recipes”. Some of the contents are
   • User-defined pruners
   • User-defined Optuna Callbacks
   • Human-in-the-loop optimization with Hyperparameter Importance
   • enqueue_trial
   • Introduction of multi-objective optimization

[crcrpar]

Due to the unexpected ReadTheDocs build failure thanks to timeout or weird memory consumption, migrating to LightGbm ~https://github.com/optuna/optuna/pull/2076~ #2088 tentatively but I wanna use some other platform such as GitHub Pages to host kind of “cool” PyTorch tutorial which I authored initially. The build time will not be problematic with GitHub Pages because we can make full use of cache in our GitHub Actions workflow.

Furthermore, we’re going to make tutorial richer by adding new recipes that are already written and made public in https://github.com/crcrpar/optuna/tree/new-recipes branch. So, even if #2088 makes it, the build time will increase. It’s worth giving a consideration to how we host our documentation, how we build and maintain our documentation including tutorial. One possibility is that we separate the tutorial directory and build it independently while work hard to guarantee that the main branch of tutorial repository is sync-ed with optuna repository.

[crcrpar]

I added all I wanted, so close.