Add example fros cross-building for embedded (bare metal) devices

New page in this section: https://docs.conan.io/2/examples/cross_build.html

Things to take into consideration:
* Using a `gcc` for the generic `arm-none-eabi` target
* The current `cpu` setting may not be enough to cover the variability of the ARM instruction sets across different devices, this covers:
* `M-profile` architecture
* different SoCs may have different CPU features, and unlike x86_64 where most distributable binaries are built against a baseline configuration (the "lowest common denominator"), in embedded/low power devices one may want to use specific CPU features that are optional
* Other compiler flags are important, e.g. with the aim of reducing code size:

see discussion https://www.reddit.com/r/cpp/comments/165fi6f/comment/jyfhb1l/