fixup docs link at Couper welcome page
<h1>Welcome to Couper.</h1>
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- <strong>Getting started: </strong>Visit our <a href="https://github.com/avenga/couper/blob/master/docs/README.md">Documentation</a> or <a href="https://github.com/avenga/couper-examples">Examples</a>.
+ <strong>Getting started: </strong>Visit our <a href="https://docs.couper.io/">Documentation</a> or <a href="https://github.com/avenga/couper-examples">Examples</a>.
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