v0.5.1

Added

• Ready for crates.io: keywords, categories, and the license text in both
  packages, a README for serde_markdown_derive, and docs.rs builds
  serde_markdown with every feature (the buffa and tokio modules).
  serde_markdown requires its own serde_markdown_derive by exact
  version.
• CI builds and tests serde_markdown and serde_markdown_derive with
  the minimum Rust version, read from rust-version.

Changed

• The minimum Rust version (rust-version) is 1.88 (it was 1.99.0), the
  oldest rustc that builds and tests the published crates.

• The workspace takes its dependencies as caret ranges instead of exact
  pins (serde = "1.0.229", was "=1.0.229"), so a consumer is not held to
  one version of serde, buffa, tokio, or the format crates. Cargo.lock
  still fixes the versions this repository builds and tests; Renovate moves
  it within the ranges.

• The README takes serde_markdown from crates.io
  (serde_markdown = "0.5") and links to GitHub by absolute URLs.

• Dependencies: at least serde_json 1.0.154, syn 3.0.9, and tokio
  1.53.3.

• Tooling: bazel_utils_* v0.4.0, Buf CLI v1.74.0, and
  bazel-contrib/setup-bazel 0.19.1 in CI. Buildifier, markdownlint, buf
  lint, and cargo audit now also run on Windows (amd64 and arm64), so every
  test but the doc tests runs on all six CI platforms; rules_rust cannot link
  doc tests on Windows (LNK1181).