docs: document Marketplace provider registration and content-for label

[lpgarzonr]

Summary

• Adds steps 5 and 6 to how-to-guides/bootstrap-provider.md documenting:
  • The three resources required for a provider to appear in the Marketplace (ProviderMetadata + APIExport + ContentConfiguration) and how the ui.platform-mesh.io/content-for label joins them — with different values per resource (see below).
  • The bind ClusterRole/ClusterRoleBinding required for the Enable button to work.
• Adds ui.platform-mesh.io/content-for to the labels table in reference/resources/metadata-catalog.md, which previously omitted it entirely.

Key finding: content-for has two different values

The content-for label serves two independent join operations with different values:

Resource	content-for value	Purpose
ProviderMetadata + APIExport	<ProviderMetadata.name> e.g. my-service	Marketplace listing — filter.go joins these two by this value
ContentConfiguration	<APIExport.name> e.g. my-service.example.com	Nav projection into consumer workspace after install — filter.go joins by the APIExport name from the APIBinding

Verified in virtual-workspaces/pkg/storage/filter.go:144 (CC lookup) and filter.go:219 (Marketplace listing), and against the github provider in local-setup (APIExport has content-for: github, CC has content-for: github.dxp.sap.com).

Motivation

A provider author following the existing docs would end up with a workspace that has all the right resources but whose card never appears in the Marketplace and whose nav nodes never project into consumer workspaces — with no error to explain why. Neither the content-for label nor the bind RBAC requirement were documented anywhere.

Note: the original gap also included a len(latestResourceSchemas) == 0 filter that blocked UI-only providers — that is fixed separately in platform-mesh/platform-mesh#332. This docs PR reflects the corrected behavior (UI-only exports with no schemas are valid once identityHash is set).

Discovered while validating the HSP → Platform Mesh extension migration end-to-end on a local kind cluster (DXPFRAME-2475).

Test plan

• Internal links resolve (no broken refs introduced)
• npm run build passes
• Prose follows style guide (sentence case headings, US English, no forbidden words, ## Related present)