docs(guide/Conceptual Overview): use exchangeratesapi.io

[bkiselka]

docs(guide/Conceptual Overview): use another exchange rate API

As fixer.io introduced an API key and thus a limitation of calls (see https://github.com/fixerAPI/fixer#readme), change https://api.fixer.io to https://api.exchangeratesapi.io instead, which is "designed and tested to handle thousands of request per second" and has "built in Fixer.io compatibility so you can keep all the libraries you already like and use daily" (from https://api.exchangeratesapi.io).

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

docs update

What is the current behavior? (You can also link to an open issue here)

The finance3.js service module in the Conceptual Overview section of the Angular documentation does not work without an API key. See #16807

What is the new behavior (if this is a feature change)?

Another another currency converter API is used instead (namely the MIT licensed https://api.exchangeratesapi.io), which does not need an API key and which is "designed and tested to handle thousands of request per second"
and has "built in Fixer.io compatibility so you can keep all the libraries you already like and use daily" (from https://api.exchangeratesapi.io). Idea is from #16137

Does this PR introduce a breaking change?

No.

Other information:

The Idea is from fixerAPI/fixer#107

fixes #16807

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[bkiselka]

I signed it! (the individual CLA)

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