Update mentions of Console

The following excerpts from the "Concepts > Effect Handlers" page are really outdated, mentioning `Console`:

https://github.com/effekt-lang/effekt-website/blob/8799da265cd742403c2fbdebd69fbb7688484cfd/docs/concepts/effect-handlers.md?plain=1#L7-L9

... and the notion of "builtin effects"

https://github.com/effekt-lang/effekt-website/blob/8799da265cd742403c2fbdebd69fbb7688484cfd/docs/concepts/effect-handlers.md?plain=1#L50-L53

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For effect handlers in particular, it might be worth it to cross-reference them with the Tour: the page Tour > Captures has a nice explanation of captures, but the Concepts > Effect Handlers seem a better intro to handling than the Tour > Effect Handlers.