doc: Add attributes to uplink message data format

[vlasebian]

Summary

References #7451 in thethingsnetwork/lorawan-stack.

Screenshots

Changes

• Add attributes to uplink message data format

Notes for Reviewers

None.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.

[KrishnaIyer]

Please add the new-in-version tag and set that to v3.34.0.

[vlasebian]

I've added the tag near the new fields, but the formatting isn't looking good. The tag formatting conflicts with the code block:

Eventually I simply added a note at the beginning of the page prefixed with the tag: