Document all exceptions we have for binaries

Though we declare that all binaries should be build from source or pulled from the trusted maven repos or Debian, we do have lots of exceptions. We should document them somewhere.

• Android SDK/NDK
• Flutter SDK, downloaded with the scripts in the srclibs
• JSC/Hermers, downloaded from npm
• Binaries from pip wheels
• Bianries from Nix cache
• Bianries from vcpkg cache
• Rust/Rustup, we may be able to use the Debian packages
• Golang, should be not needed after the fdroidserver upgrade if finally
• Nodejs, maybe not needed after the fdroidserver upgrade if finally
• Some other compilers/build tools not packaged in our very old Debian
• Maybe more?