Markdown doc creation should only create links to defined arguments

Describe the bug
I've updated my template to use semconv 1.37.0 and now my markdown docs are being generated with an Entities link, but there are no entities in my attributes.yaml.
This is just resulting in a broken link.

To Reproduce
Steps to reproduce the behavior:

1. Clone the sample:

   git clone git@github.com:jerbly/weaver-example.git
   
   cd weaver-example
   

2. Update the registry_manifest.yaml to use semconv 1.37.0.

3. Generate the markdown. I've used docker, by running:

   docker run --rm \
       --mount 'type=bind,source=./model,target=/home/weaver/model' \
       --mount 'type=bind,source=./docs,target=/home/weaver/docs' \
       otel/weaver:v0.17.1 \
       registry generate \
       -r model \
       --templates "https://github.com/open-telemetry/semantic-conventions/archive/refs/tags/v1.37.0.zip[templates]" \
       markdown docs
   

4. When the docs are created, check the docs/README.md and you will find a new link at the bottom of the file:

   - [Entities](entities/README.md)
   

   But there is no entities/README.md file, resulting in a broken link.

Expected behavior

The markdown docs should create a markdown only with what I have defined in the attributes.yaml.

Reproduce the issue with the linked weaver-example, semconv 1.37.0, and the registry generate command, then inspect the markdown generation path that creates docs/README.md. Compare the generated links with the definitions in attributes.yaml; done means no entities/README.md link is emitted when no entities are defined.