doc: Update JSON device file reference

[vlasebian]

Summary

Update the JSON device file reference documentation for bulk device imports.

Closes #1468.

Changes

• Remove fixed-table class from the device JSON table div.
• Change supports_join field from Always required to No (optional), defaults to false (ABP) if not set.
• Add session.keys.session_key_id field documentation, required when importing devices via the Console.

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.