Vspec link in AppManifest

[mml5bg]

Describe your changes

This PR adds a link to Vspec to AppManifest.json.

Issue ticket number and link

AB#12509

Checklist - Manual tasks

• [ ] Examples are executing successfully
• [ ] Created/updated unit tests. Code Coverage percentage on new code shall be >= 80%.
• [ ] Created/updated integration tests.
• [ ] Devcontainer can be opened successfully
• [ ] Devcontainer can be opened successfully behind a corporate proxy
• [ ] Devcontainer can be re-built successfully
• [ ] Extended the documentation (e.g. README.md, CONTRIBUTING.md, Velocitas)