Adds `.github/dependabot.yml` enabling weekly cargo update PRs for the
root workspace, which produces the Rust binaries we ship: the Node.js
and Python native extensions. The `rust/lancedb` library crate shares
the same lockfile — its consumers pick versions themselves, but bumping
transitive deps here keeps the shipped binaries current.
Also removes the misleading `exclude = ["python"]` line from the root
`Cargo.toml`: `python` is listed in `members`, and `cargo metadata`
confirms it's a workspace member, so the exclude was dead code that
implied the opposite.
Minor/patch updates are grouped to reduce PR noise.
Part of #3292. Only covers the cargo ecosystem; pip, npm, and
github-actions can follow.
