Documentation about Functional algorithms needs to be updated

The documentation about the Gaudi Functional usage in Key4hep ([see here](https://key4hep.github.io/key4hep-doc/developing-key4hep-software/WritingAlgorithms.html) is still using `Gaudi::Functional` algorithms, when they should be pointing towards the `k4FWCore` variants of them. At least they should be the default that people go to. There is room for also mentioning that `Gaudi::Functional`s also work, but there the differences need to be explicitly stated.

Start with the WritingAlgorithms.html page linked in the issue and review its current Gaudi::Functional examples. Identify the corresponding k4FWCore variants to present as the default, then state the differences for Gaudi::Functional alternatives. Done means the documentation points newcomers to k4FWCore while accurately covering both options.

Written by the indexing model from the issue text.