Add SAP SuccessFactors Employee Central connector catalog docs (wso2-connectors)

[RDPerera]

Summary

• Adds connector catalog docs for the 21 SAP SuccessFactors Employee Central connectors (overview, setup guide, action reference, example, plus screenshots) under en/docs/connectors/catalog/hrms/sap.successfactors.*
• Same content as Add SAP SuccessFactors Employee Central connector catalog docs #662 (which targets 5.1.x), adapted to the wso2-connectors site:
  • sidebar categories use this branch's catalog/... doc ids, inserted alphabetically after SAP Signavio
  • all 21 connectors are registered in the Connector Catalog page (category HRMS, vendor SAP, version 2.0.0)
  • images converted to ThemedImage, matching the rest of the catalog after Convert markdown images in connector docs to ThemedImage #720
  • the Employee Central Advances "Try it yourself" link now points at the sample directory that exists in integration-samples

Test plan

• All 21 sample links resolve to directories in wso2/integration-samples (Add sample projects for SAP SuccessFactors Employee Central connectors integration-samples#165)
• All 126 image references resolve to files added in this PR; no markdown image syntax left
• Sidebar: 21 new categories, no connectors/-prefixed ids; catalog registry: 21 new entries, icon URLs verified

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