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

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 ).
The idea is from https://github.com/fixerAPI/fixer/issues/107

fixes  #16807

Closes #16808