doc: Add New error while claiming TTIGPro

[sravankumar985]

Summary

Add new error gateway subscription not attached and active while claiming The Things Indoor Gateway Pro. This is to add under new heading Troubleshooting in the TTIGPro page

Error Details

"error":"error:pkg/deviceclaimingserver:claim gateway (claim gateway)",
"error_cause":"error:unknown:unknown (gateway subscription not attached and active)",

Screenshots

New:

Changes

Added new error gateway subscription not attached and active while claiming TTIGPro gateway and explanation.

Notes for Reviewers

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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.