[MINOR][DOCS] Add license header at docs/_plugins

[HyukjinKwon]

What changes were proposed in this pull request?

This PR adds license header to docs/_plugins files.

Why are the changes needed?

To comply Apache License 2.0

Does this PR introduce any user-facing change?

No, dev-only.

How was this patch tested?

Existing CI should verify it e.g., linter.

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

No.

[dongjoon-hyun]

+1, LGTM.

[nchammas]

Should we update the RAT excludes?

spark/dev/.rat-excludes

Line 20 in c0ff0f5

docs

I believe this is why the build did not catch these missing licenses.

[HyukjinKwon]

I think we should probably update.

[nchammas]

FYI - I am checking upstream with the RAT devs to see if we can improve how the tool works. Having RAT scan docs/ triggers a lot of spurious failures that are tedious to sort out.

Because we use the CLI and not the Maven plugin (some history here), RAT does not automatically look at our .gitignore files. And the RAT exclude file patterns are regular expressions and not wildcard expressions, which I find confusing. (I think I am not alone on this point, judging from some of the lines in .rat-excludes.)

I am following RAT-323 and RAT-352 to see if RAT will get some usability improvements for the CLI in the near future.

[dongjoon-hyun]

+1, LGTM.