docs: Add basic tutorials

[luator]

Description

Add two basic tutorials for grid_search and hp_optimization.

The goal of these tutorials is to get new users started. As such, they are intentionally minimalistic and do not explain all the available options. They should enable beginners to get cluster_utils running fast and from there allow them to explore more options step by step (though for that much more documentation needs to be written eventually).

I put them in the "Basics" section for now, but if more tutorials will be added, it might make sense to add a dedicated "Tutorials" section.

Rendered pages

Here is what the tutorial pages look like when built locally:

• Tutorial: Basic Grid Search
• Tutorial: Basic Hyperparameter Optimization

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Addresses part of #50.

[luator]

@nitinjain96 No code change here, but if you have time for it, it would be great if you could read through the tutorials, to check if everything is easy to understand (from the perspective of a new user).