Add "zeroth" tutorial as an extended version of the README for the first steps with Trixi

[sloede]

@torrilhon suggested to add a "zeroth" tutorial that guides users through the first steps of using Trixi, e.g.,

1. Install Trixi and additional required packages
2. Run a first simulation
3. Visualize it
4. Get an elixir from GitHub
5. Run it
6. Modify the elixir by, e.g., adding your own initial condition (advection with two Gaussian pulses?)

Basically an extended version of the stuff in the README, geared towards new(er) Julia users.

[starlightknown]

Hello, I would like to contribute to this issue, is there any style guide to follow?

[ranocha]

We have a brief style guide in our docs at https://trixi-framework.github.io/Trixi.jl/stable/styleguide/

You are welcome to prepare a PR 👍 We will review it and give you feedback if another approach/style would be preferred. You can of course also start with one part in the PR, ask for feedback, and finish it based on the comments you receive.

[sloede]

@starlightknown it would be great if you could let me know if you are actively working on this yet or not? If not, we have a new student starting to work with Trixi who'd be ideal to work on this.

[JoshuaLampert]

Completed in #1343.