ci: add Dependabot config for shipped Rust binaries (#3300)

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.

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+
+# Scope: the root Cargo workspace, which produces the Rust binaries we
+# ship to users (the Node.js and Python native extensions). The
+# `rust/lancedb` library crate shares the same lockfile; its consumers
+# pick their own dependency versions, but bumping transitive deps here
+# keeps the binaries we ship current.
+updates:
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 10
+    groups:
+      rust-minor-patch:
+        update-types:
+          - minor
+          - patch