Ignore CMakeUserPresets.json in VCS and provide templates in README.md

According to CMake Docs, CMakeUserPresets.json is designed for developers to specify their own local build details. Therefore, it shouldn't be added to VCS.

Besides, the current install/build instructions of qt-creator is running a cmake command with a series of arguments in CLI, which is quite troublesome. See: 

• Build/Install instructions on Linux and macOS
• Build/Install instructions on Windows

Therefore, I suggest that qt-creator:

• Add CMakeUserPresets.json in .gitignore.
• Provide templates of CMakeUserPresets.json in README.md.

so that developers may be able to build qt-creator locally with the help of some IDEs, such as VSCode, CLion...etc, out-of-the-box. And I believe this method of building qt-creator project is more convenient.

For example, the following screenshot is my demonstration of using CMakeUserPresets.json with VSCode (CMake Tools) to build the qt-creator.