Update wiki links

[dyfer]

As a follow-up to #247 I've made following changes:

• in the top navigation bar I replaced Wiki link from "Pages" to SuperCollider wiki (*)
• on the main page, I updated the link to the Wiki so that it points to SuperCollider wiki
  • I've also removed links to the old wikis, as they are now linked from the main SuperCollider wiki

(*) I'm not fluent in Jekyll and I'm not sure if I created the "external link" correctly - it seems hack-y... I've added a new layout that refreshes the page, pointing to the external URL. Is there a better way to add an external link to navigation?

[dyfer]

Thanks @gusano ! This was super helpful, I think it's better now.