[MINOR][DOCS] Add docs/_generated/ to .gitignore

### What changes were proposed in this pull request?

This PR adds the auto-generated content `docs/_generated/` to .gitignore

### Why are the changes needed?

vcs improvement

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

existing tests

### Was this patch authored or co-authored using generative AI tooling?

no

Closes apache#46178 from yaooqinn/minor.

Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

.gitignore

@@ -50,6 +50,7 @@ dev/create-release/*final
 dev/create-release/*txt
 dev/pr-deps/
 dist/
+docs/_generated/
 docs/_site/
 docs/api
 docs/.local_ruby_bundle