docs: improve INTRO-cmake.md

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Background

When going through the SDL_image CMake installation instructions, the user will run into some problems.

1. There's no step for cloning the SDL_image git repository into the vendored directory.
2. "vendored/SDL" is also required and referenced in the CMakeLists.txt file. Arguably cloning the SDL git repository should also be included in the instructions.

Changes

• Added steps for cloning the SDL and SDL_image repositories into the vendored directory

Comments

This change will also align this file with the SDL repository INTRO-cmake.md which includes the git clone steps.