Document the rough steps to get Slint working in C++ MCU environments

[tronical]

We recently had a user who managed to get Slint running on an STM32U5 with C++. We ended up sorting everything out on Mattermost, but it became apparent that the documentation is missing a guide here:

• Figure out the right Rust target and then set -DRust_CARGO_TARGET= (for example to thumbv7em-none-eabihf).
• Set -DSLINT_FEATURE_FREESTANDING=ON to not require std.
• Set -DBUILD_SHARED_LIBS=OFF to build a static lib (no shared libs on mcu).
• Set -DSLINT_FEATURE_RENDERER_SOFTWARE=ON
• Set -DDEFAULT_SLINT_EMBED_RESOURCES=embed-for-software-renderer

[ogoffart]

imho we still need a better guide on how to implement a platform for an MCU. https://slint.dev/releases/1.3.2/docs/cpp/api/namespace_slint__platform is not too bad, but could be improved.