Add scala 3 contributing guide - MVP

[bishabosha]

this is the initial version of the Scala 3 contributing guide, optimised for dotty sprees.

[sjrd]

Reviewed "Procedures" so far.

[sjrd]

And that's the review for the Architecture.

[vincenzobaz]

The other reviewers probably have a lot more knowledge than me on this topic.
I wish I had this before my first spree!

I left some minor comments, but the guide looks very good and it contains a lot cool tips on testing. For the Spree I suggest to create a resume or shorter version, because there is a lot of material!

[vincenzobaz]

Earlier you mentioned a few phases (typer, parser) and @sjrd mentioned erasure...
Here I only see parser and typer. Where are the other phases?

[vincenzobaz]

Do other people in the team want to share their editor setup? I imagine most of them use VSCode so it would be nice to have helpers for it as well

[anatoliykmetyuk]

Thanks everyone for the review and suggestions! I've applied the most straightforward ones while leaving more in-depth one for later. This is because we have only a couple days left until the spree, so we need to have something soon. Let's return to this PR in the future iterations of the development of this guide.