Rename "beginner" labels as "introduction"

[m09]

As discussed in #55, we should rename beginner to introduction.

[vmarkovtsev]

So actually I think that we should revisit this, because there will be some people for sure who will say that some paper is not an introduction, they will open an issue and we will have to discuss everything again.

I would rather add a second label and see how many of each we get.

[m09]

I agree on the ambiguity of the label. Should we forgo introduction completely then? Or establish a better typology with labels such as bibliographic-review, experimential, theoretical, etc?

[bzz]

Usually, in decisions like this - to pick a better name or option what helps me is to recall what problem are we trying to solve with it.

[vmarkovtsev]

@bzz I think that the core problem was #55 (comment)