Improve documentation regarding cross-build

Current documentation regarding cross-build could be improved:

- how to use current cross-compilers recipes packaged in conancenter: `android-ndk` & `emsdk` for example.
- how to write a recipe for a cross-compiler so that it can be used in a host profile and be robust to all build systems.
- how to write a custom host profile for a system installed cross-compiler without depending on a cross-compiler recipe, but something more complex that raspberry example which is too simple with its 3 env vars:
- Explain which common builldenv vars may have to be defined (`CC`, `CXX`, `AR`, `AS`, `LD`, `RANLIB`, `NM`, `STRIP`, `OBJCOPY`, `OBJDUMP`, `READELF`, `ADDR2LINE`, `ELFEDIT`, `CHOST`, `SYSROOT` etc) and conf (`tools.build:compiler_executables`, `tools.cmake.cmaketoolchain:user_toolchain`, `tools.build:sysroot`).
- Avoid mixing compiler path/name with flags in `CC` and `CXX`, prefer to define these flags in `tools.build:cflags`/`tools.build:cxxflags`, because conan may not be robust to spaces in `CC`/`CXX`/`tools.build:compiler_executables`