ci: Treat rustdoc warnings as errors in validate target

Broaden the existing 'cargo doc --lib' in the Makefile validate
target to '--workspace --no-deps --document-private-items' and
set RUSTDOCFLAGS='-D warnings' so that any rustdoc warnings
(broken links, unclosed HTML tags, bare URLs, etc.) will fail
CI.

Assisted-by: OpenCode (claude-opus-4-6)
Signed-off-by: John Eckersberg <jeckersb@redhat.com>

Author: jeckersb

Makefile

@@ -123,7 +123,7 @@ validate:
 	(cd crates/ostree-ext && cargo check --no-default-features)
 	(cd crates/lib && cargo check --no-default-features)
 	cargo clippy -- $(CLIPPY_CONFIG)
-	env RUSTDOCFLAGS='-D warnings' cargo doc --lib
+	env RUSTDOCFLAGS='-D warnings' cargo doc --workspace --no-deps --document-private-items
 .PHONY: validate
 fix-rust:
 	cargo clippy --fix --allow-dirty -- $(CLIPPY_CONFIG)