Improve doc: Link to this repo and state latest released version on toolkit doc page at scala-lang.org, and include concrete example of using directive

[bjornregnell]

I think it would be helpful to state the latest version released of the toolkit on the doc page here:
https://docs.scala-lang.org/toolkit/introduction.html

And also link to its home on github: https://github.com/scala/toolkit

As a user I want to know how to pin down the latest/default version in my using directive //> using toolkit x.y.z

[bjornregnell]

I also think it would be helpful if the Introduction page included an actual example of how to use the toolkit as a using-directive both with default and with a specific version.

//> using toolkit default

or pinned:

//> using toolkit 0.6.0

[adpi2]

Thanks for the suggestions! I fully agree and support these changes.