Add CMake option to control building of examples

[kaniteru]

Hello,

I encountered an issue when using FetchContent to include WebUI in another CMake project.
In this case, CMAKE_SOURCE_DIR does not point to the WebUI root directory, which results in errors.

Additionally, the example projects are always built, even when they are not needed.

So, I made a few small changes to fix this:

• Replaced CMAKE_SOURCE_DIR with CMAKE_CURRENT_SOURCE_DIR
• Added a WEBUI_BUILD_EXAMPLES option to control whether example projects are built

Thank you.

[AlbertShown]

Thank you @kaniteru for the CMake improvements.