Add precompiled NIF artifacts via rustler_precompiled (#27)

[kipcole9]

Add precompiled NIF artifacts via rustler_precompiled (#27). This is intended to make installation easier for most users while preserving the manual build process option for anyone that wants it.

Consumers on aarch64/x86_64 macOS, aarch64/x86_64 Linux glibc, and
x86_64 Windows MSVC no longer need a Rust toolchain — Ortex downloads
a prebuilt NIF from the GitHub release. Source build remains the
fallback for unlisted targets and via ORTEX_BUILD=true.

@lawik requested Nerves cross-compilation support too and I'm
willing to tackle that if this PR is accepted.

Changes

• lib/ortex/native.ex: use RustlerPrecompiled (handles both download
  and force_build paths). Ortex.Util.copy_ort_libs still runs after
  source build to stage libonnxruntime when the toolchain produces a
  dynamically-linked NIF.
• mix.exs: add :rustler_precompiled, make :rustler optional, ship
  CHANGELOG.md and the checksum file in the package.
• .github/workflows/release.yml: matrix builds 5 targets × 2 NIF
  versions on tag push via philss/rustler-precompiled-action,
  uploads to a draft release.
• .github/workflows/ci.yml: forces source build via ORTEX_BUILD=true
  so PR validation isn't dependent on release artifacts existing. Preserves
  the existing behaviour.
• checksum-Elixir.Ortex.Native.exs, RELEASE.md, CHANGELOG.md,
  README.md: pinned-checksums placeholder, release runbook,
  changelog entry, precompile target list and source-build
  instructions.

Verified end-to-end on Debian Bookworm with no Rust toolchain via a
mix deps.compile + tinymodel.onnx inference test in Docker.

Updated Release process

Since we generate precompiled artefacts on tag push, the release process is a bit different and documented in RELEASE.md. It follows the standard process for other rustler_precompiled projects. In short:

1. Bump version
2. Tag and push the tag (triggers workflows/release.yml)
3. Promote the draft release to published in Github after the artefacts are built.
4. Generate the checksum file with ORTEX_BUILD=true mix rustler_precompiled.download Ortex.Native --all
5. Commit the checksum file and force-move the tag.