Add Introduction section to docs

[tbrowder]

Add an Introduction "section"

The current online docs need a short introductory para introducing the page.Include a link to the online version of the ‘Raku.guide’ [https:/raku.guide].

[coke]

https://docs.raku.org/introduction does exist, but happy to consider improvements!

[tbrowder]

My bad. Closing.

[coke]

No, this one is fine, we can use better text on the intro page!