WIP: Add services as applications explanation and example

[gmoigneu]

Why

As we want to promote running services as applications, this PR adds a section in /add-services to explain how to do this and list the available examples.

What's changed

/add-services/index.md

Where are changes

Updates are for:

• platform (sites/platform templates)
• upsun (sites/upsun templates)

[github-actions]

Your Platform.sh environment has successfully deployed. 🚀

See the changed pages:

Platform.sh docs

Upsun docs

• https://docs.upsun.com.add-apps-to-services-3h4fgci-ucq44jg6ofare.eu-5.platformsh.site/add-services/user-defined-services.html
• https://docs.upsun.com.add-apps-to-services-3h4fgci-ucq44jg6ofare.eu-5.platformsh.site/get-started/AI.html
• https://docs.upsun.com.add-apps-to-services-3h4fgci-ucq44jg6ofare.eu-5.platformsh.site/add-services/user-defined-services/chroma.html
• https://docs.upsun.com.add-apps-to-services-3h4fgci-ucq44jg6ofare.eu-5.platformsh.site/add-services/user-defined-services/qdrant.html

[fabpot]

I would only explain that we also have HTTP-based service that are configured via our multi-app feature and not explain how on can do it for their own (which should be documented separately).