Rename Project to Errors C++

[threeal]

This pull request implements the following changes to rename the project to "Errors C++":

• The Errors package (formerly named Error) is moved to the root directory, making it the sole C++ package.
• Documentation is adjusted to cover only the Errors package.
• The CI workflow is updated to build only the Errors package.
• CMake project, targets, and headers are renamed with the errors prefix.
• The project URL is updated to threeal/errors-cpp.