Simplify "Connect an existing account"

[soxofaan]

documentation/authentication/index.md

Lines 24 to 27 in c1d4960

	If you already have done this in the past, you can skip this step and
	continue with [joining openEO Platform](#join-openeo-platform).
	Otherwise, please go to <https://aai.egi.eu/signup> to become member of the *EGI User community*.

I think the docs can be simplified a bit by (for example) removing the part "If you already have done this ... Otherwise", and let EGI handle the "you are already registered" use case.

Unfortunately the user experience of doing https://aai.egi.eu/signup when you are already registered is bad (slow and cryptic error message at the end) . I have ticket about that here: https://ggus.eu/index.php?mode=ticket_info&ticket_id=154418

At the moment I would keep things as-is, but when EGI UX is improved, and depending on how it looks, we might want to fine-tune this.