[Feature Request]: Validate that first index.md has no children in toc.yml

[theletterf]

Prerequisites

• I have searched existing issues to ensure this feature hasn't already been requested
• I have tested using the latest version of docs-builder

What problem are you trying to solve?

When adding children to index.md in a toc.yml file, the first navigation entry gets duplicated.

Sample snippet:

project: 'APM Java agent reference'
toc:
  - file: index.md
     children:
       - file: setup-javaagent.md
       - file: setup-attach-cli.md
    ...

Example result is a duplicated, nested "APM Java agent reference" entry.

Proposed Solution

We could validate that the first index.md file doesn't have children.

Examples and Research

No response

Alternative Solutions

No response

Additional Context

No response

How important is this feature to you?

Nice to have