Add reference docs for kro

[frewilhelm]

What this PR does / why we need it:

Adds reference documentation for the kro service provider so users discovering OpenControlPlane can see kro alongside the other supported services.

• New CRD reference page at docs/reference/services/kro.md documenting the Kro Kind (kro.services.openmcp.cloud/v1alpha1), with a usage example and a CRDViewerCompact pointing at the upstream CRD manifest.
• New tile in the reference overview (docs/reference/00-overview.md) and a new link to the service-provider-kro repository under "CRD definitions".
• New ecosystem card on docs/users/ecosystem.md introducing kro as an upstream project.
• Adds dedicated tower icons tower_ocm.png and tower_kro.png.
• OCM reference page: removes the now-stale exampleUrl and switches the namespace in the usage example to a generic my-namespace.

Which issue(s) this PR fixes:
Related to open-component-model/ocm-project#1011

Special notes for your reviewer:

• Adds object-fit: contain on .crd-header-icon so the new non-square tower assets render cleanly.

Release note:

Added reference documentation and ecosystem entry for the kro service provider.