Update about.md in About Slices concept

[Inkognitoo]

Add information about merging slices with append

[github-actions]

Dear Inkognitoo

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_{Automated comment created by PR Commenter 🤖.}

[andrerfcsantos]

Thanks for this!

I think this is a great change since the exercise directly requires this and you kept it brief.

You made this change in about.md which is fine, but this file is only shown when the student completes the exercise/concept, which is late to show this information. My suggestion is to also make this change in the concept's introduction concepts/slices/introduction.md and in the exercise's introduction exercises/concept/card-tricks/.docs/introduction.md just so the student is shown this explanation before attempting the exercise.

The exercise's introduction also includes a simpler example on how to use append with multiple arguments without using the ... notation. I think we should also include that in this explanation:

a := []int{1, 3}
b := append(a, 4, 2) // b == []int{1,3,4,2}

I'll let you choose how you want to incorporate this example in the explanation, but I'm thinking of maybe having a paragraph + code sample explaining this simple example first and then another paragraph + code sample explaining the ... notation. And to be clear, this extra example should be included in the 3 files, not just in the exercise's introduction.

[andrerfcsantos]

@Inkognitoo Let us know if you are still interested in working on this. It's totally fine if you are not, we can finish this for you. If you are still interested in working on this feel free to take your time, there's no rush.

[andrerfcsantos]

It has been a while. Since the changes of this PR were small and the suggestions easy to implement, I made the final changes to finish the PR.

Thanks!