Add a link to OpenAPI spec in UI

### Current Behavior

Finding the API spec requires googling and then finding out that the /api/swagger.json mentioned in places no longer exists and the 404 returned is not a deployment problem but a feature. The new endpoint is /api/openapi.json and /api/openapi.yaml.

### Proposed Behavior

add a link to the menu pointing to the API spec, for example like in Defect Dojo:


might as well add a link to the docs there

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this enhancement was already requested

Start by locating the frontend menu where navigation links are rendered and check how external or API links are presented. Add links to /api/openapi.json, /api/openapi.yaml, and the API documentation, then verify each link is visible in the menu and opens successfully.

Written by the indexing model from the issue text.