GH-39600: [R] Add trademark attribution to pkgdown site footer

[thisisnic]

Rationale for this change

Per ASF policy, documentation pages should include trademark attribution in the footer. See #39461 for the broader effort.

What changes are included in this PR?

Add Apache trademark attribution text to the R pkgdown site footer.

Are these changes tested?

Documentation only. Can verify by building pkgdown site locally.

Are there any user-facing changes?

Footer now includes trademark text matching the main Arrow website.

---

This PR was generated by Claude. All outputs were reviewed and confirmed before pushing.

• GitHub Issue: [R] Use correct attribution in the footer of pkgdown site #39600

[thisisnic]

@github-actions crossbow submit preview-docs

[github-actions]

⚠️ GitHub issue #39600 has been automatically assigned in GitHub to PR creator.

[github-actions]

Revision: 5e14191

Submitted crossbow builds: ursacomputing/crossbow @ actions-3bc0f1949a

Task	Status
preview-docs

[thisisnic]

View preview: https://s3.amazonaws.com/arrow-data/pr_docs/49332/r/index.html

[jonkeane]

I'm not a huge huge fan of the formatting, at a minimum I wish it had a bit more whitespace between the older version bit and the legal text.

I have some suggestions for making it a separate component which might help some, but we maybe nee do <div style="...">...</div> to make this a little more separate from the current footer.

Alternatively: if it were the full length of the screen slightly above the current footer that might make it stand out a little bit less (in a good way)?

[jonkeane]

Suggested change

	older_versions: |
	[Older versions of these docs](https://arrow.apache.org/docs/r/versions.html)
	<br><small>Apache Arrow, Arrow, Apache, the Apache logo, and the Apache Arrow project logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</small>
	older_versions: "[Older versions of these docs](https://arrow.apache.org/docs/r/versions.html)"
	legal: |
	<small>Apache Arrow, Arrow, Apache, the Apache logo, and the Apache Arrow project logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</small>

We can have multiple components in the footer

[jonkeane]

Suggested change

	right: built_with
	right: [built_with, legal]