Rework the API links order, name, and targets

[MakisH]

Follow-up of #718:

• Renames Location to Source code
• Moves it to the bottom of each section, to not confuse users
• Changes the C++ API "location" from https://github.com/precice/precice/tree/main/src/precice/Participant.hpp to https://github.com/precice/precice/tree/main/src/precice. The intention here is to have an entry point to find the rest (and was already pointing to Participant.hpp before Restructure API page #718).

[fsimonis]

I don't think moving this is a good solution either.
IMHO these should all simply link to the repository and relevant information should be in the repo Readme. If someone wants to look at source, then they should look at the repository.