doc: Add new API error

[sravankumar985]

Summary

Added new error "neither_user_nor_organization" error heading in the API troubleshooting guide.
As there is an explanation for a similar error, just added the error next to the existing error.

Error

{
    "code": 7,
    "message": "error:pkg/identityserver:neither_user_nor_organization (caller is neither a user nor an organization)",
    "details": [
        {
            "@type": "type.googleapis.com/ttn.lorawan.v3.ErrorDetails",
            "namespace": "pkg/identityserver",
            "name": "neither_user_nor_organization",
            "message_format": "caller is neither a user nor an organization",
            "correlation_id": "xxxxx",
            "code": 7
        }
    ]
}

Screenshots

New:

Changes

Added new error heading alone, as there is an explanation for a similar error.

Notes for Reviewers

...

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.