Add more guidelines for labelling code examples for Scala 2/3

[julienrf]

Also, apply the recommended approach to a couple of existing pages.

Before:

[bishabosha]

You kept the "Scala 2 and 3" tab on ca-given-using-clauses.md which is a bit strange.

or is this intentional because its "otherwise stated"?

[julienrf]

I kept that one because it shows what the code looks like when we don’t use given/using. I also kept a similar code example with “Scala 2 and 3” in the Union Types page, which shows an alternative to union types.

But I agree that in the given/using page I could remove the code tab, though, if you think it is confusing.