feat(model-ad): add API docs site

[tschaffter]

Changelog

• Create API docs site project model-ad-api-docs (adapted from openchallenges-api-docs)
• Update the Docker Compose files to deploy the API docs site
• Update Caddy config to forward traffic to the API docs site

Preview

Start the stack with Docker Compose, then navigate to localhost:8000/api-docs to view the API docs site.

[tschaffter]

@hallieswan Adding the API docs site to the stack requires to update the ~apps/model-ad/app/.env~ based on the example file.

EDIT: I meant the configuration of the apex project (Caddy)