docs: current bilingual build guide; retire the C++/CMake one

docs/build.md + docs/zh/build.md rewritten for the Rust workspace:
project-built FFmpeg 8.1 (.cargo/config.toml presets FFMPEG_DIR),
vendored static OCIO on Linux/macOS vs MSYS2 dynamic OCIO on Windows
(with the OCIO_INSTALL_DIR/OCIO_RS_LINK env), the Windows GNU toolchain
requirements (MSYS2 Rust, RUSTFLAGS=-C link-args=-lmsvcrt for the
mingw-w64 _assert forwarding, unset INCLUDE/LIB), Linux audio dev
packages and xvfb headless testing, container packaging, and a
troubleshooting section. The macOS-only guides gain a deprecation
pointer. Also correct two stale comments in tooling/install-deps.sh
(FFmpeg is built by tooling/ffmpeg/build-ffmpeg.sh, not by cargo).

tooling/install-deps.sh

@@ -18,8 +18,8 @@
 # Installs the system dependencies of the Oak Rust workspace:
 # the free-license external codec/filter libraries FFmpeg is configured
 # with (see crates/oakcodec/Cargo.toml), plus the build tools. FFmpeg
-# itself is built from source by cargo (ffmpeg-next `build` feature) and
-# is NOT installed here.
+# itself is built from source by tooling/ffmpeg/build-ffmpeg.sh (which
+# installs into .cache/ffmpeg) and is NOT installed here.
 #
 # Supported: Homebrew (macOS), MSYS2 UCRT64 (Windows), Debian/Ubuntu,
 # Fedora, Arch. Run it yourself — nothing in the build invokes it
@@ -102,4 +102,4 @@ case "$(uname -s)" in
 		;;
 esac
 
-echo "Done. 'cargo build' now compiles FFmpeg 8.1 from source with these libraries."
+echo "Done. Now run tooling/ffmpeg/build-ffmpeg.sh once, then 'cargo build'."