Add spock quickstart documentation

[RSchlenker]

References: #1162

Thanks @antrew & @kiview
Created at hackergarten @ gr8conf 2019.

[kiview]

Of course there is quite some content overlap with the JUnit4 quickstart documentation.
We decided to do this, to give Spock users the complete information necessary without looking at the JUnit4 quickstart.

Is it possible to include text snippets from other markdown files?

[rnorth]

This looks great, thanks! I think it's also conceptually close to @bmuschko's #1416 - it's good to move in the same direction.

Re:

Is it possible to include text snippets from other markdown files?

Could you clarify further? Are you thinking about reducing duplication between the *_quickstart.md files?

[kiview]

Could you clarify further? Are you thinking about reducing duplication between the *_quickstart.md files?

Exactly this. So the introductory text is the same, but it makes sense to have everything right there on this single page, it will give a new user the best quickstart experience.

[rnorth]

I don't think we can do straight includes in markdown at present, but that's maybe a future improvement we can make.

[rnorth]

Thank you for the contribution @RSchlenker!

[kiview]

Merged, thanks a lot @RSchlenker and @antrew!