See BACKENDS and EXAMPLES files in the docs/ folder.

Backends = Helper code to facilitate integration with platforms/graphics api (used by Examples + should be used by your app).
Examples = Standalone applications showcasing integration with platforms/graphics api.

Once Dear ImGui is running (in either examples or your own application/game/engine),
run and refer to ImGui::ShowDemoWindow() in imgui_demo.cpp for the end-user API.