fix(docs): publish complete API reference on docs.rs

[dekobon]

What

Make docs.rs publish the complete API reference and link it
prominently from the book.

docs.rs was building with default features only. Since default = ["all-languages"] excludes vcs, the entire feature-gated vcs
module (change-history metrics, #328) was silently missing from the
published API reference — the reference itself is 100% documented
(missing_docs clean, CI-gated), but the published copy dropped a
whole public surface.

Changes

• Cargo.toml — add [package.metadata.docs.rs] with
  all-features = true and rustdoc-args = ["--cfg", "docsrs"], so
  docs.rs documents every public surface including vcs.
• src/lib.rs — gate feature(doc_cfg) on --cfg docsrs and add a
  doc(cfg(feature = "vcs-git")) badge on pub mod vcs, rendering a
  per-item "Available on crate feature vcs-git" note. The module doc
  also points users at the vcs umbrella feature. The docsrs cfg is
  inert on stable builds.
• Makefile — add doc-check-docsrs to reproduce the docs.rs build
  locally on nightly (--cfg docsrs) for pre-release verification;
  skips cleanly when nightly is absent. Wired into .PHONY and
  make help.
• big-code-analysis-book/src/README.md — add a prominent API
  reference callout on the landing page and clarify the existing "API
  docs" section. (An mdBook SUMMARY.md sidebar entry was tried first
  but mdBook 0.5.3 rejects external URLs as chapter entries.)
• CHANGELOG.md — ### Fixed entry under [Unreleased].

Why the book was the wrong place for the reference

The request began as "add API documentation to the book." For the Rust
library API the book is the wrong home: rustdoc/docs.rs is canonical,
auto-generated, and already CI-gated — duplicating signatures into
Markdown guarantees drift. The book already deferred there. The real
defect was that docs.rs shipped an incomplete copy; this fixes that and
makes the link unmissable rather than duplicating the reference.

Validation

• make pre-commit — pass.
• make doc-check-docsrs (nightly, --cfg docsrs, -D warnings) —
  clean; badge renders and the vcs surface is exercised.
• Stable make doc-check and cargo clippy --all-features -- -D warnings — clean (docsrs is a well-known cfg in cargo 1.95, so no
  unexpected_cfgs; badges stay inert off docs.rs).
• No metric computation touched — no snapshot drift.