Improve Android cross-compile scenario by covering extra cases

Greetings, this issue is a follow-up over #3351 and https://github.com/conan-io/conan-center-index/issues/24915

The current documentation https://docs.conan.io/2/examples/cross_build/android/ndk.html#examples-cross-build-android-ndk shows how to cross-compile to Android by using a NDK installed in the system in a very regular case using CMake. It works for the example, but when exploring other cases, like trying to build using autotools or not well prepared project, it may fail.

It would be nice updating the documentation by adding some hints, as cross-building is always a harder scenario and not always work at first try, being frustrating for users.

- Add `android-ndk/r25c` as tool-requirement, as alternative. That CCI package configures everything already, including ndk-path.
- Complement the configuration with compiler path (tools.build:compiler_executables) to cover other scenarios when ndk path is not enough (e.g. Boost recipe using B2)

/cc @cjserio @neobrain