Update the C++ API location / URLs

[MakisH]

The "Location" of the C++ API currently links to Participant.hpp. This is the direct link to the API (so, easy for quick access), but it is not the header that we want people to include. This is a bit confusing.

This PR:

• As "location", links to the directory https://github.com/precice/precice/tree/main/src/precice, similarly to the C/Fortran bindings
• Adds a link to "header" (precice.hpp)
• Keeps the link to "API" (Participant.hpp)
• Fixes the locations of the C/Fortran bindings to remove the GitHub URL parts from the locations

From a discussion with @dabele.

@fsimonis what do you think?

[fsimonis]

It still looks mixed to me.

Maybe the issue is the heading Location. If we change that to Repository, then it's clear what we mean and C++, C and Fortran would point to precice/precice.

Instead of the actual location, we could point to the API docs instead.

Not all bindings have a clear location, as they may contain out of multiple files and folders.

Also doesn't this collide with #718?

[MakisH]

Right, I forgot that #718 was still open. I ported the changes there, which should cover all points.