feat: PaaS Native add Opensearch documentation

[renaudhager]

This pull request adds a new guide for setting up Opensearch with Shopware in the PaaS product documentation. The guide provides step-by-step instructions for enabling Opensearch in the application.yaml file and explains the required post-enablement actions.

Documentation additions:

• Added a new markdown guide, opensearch.md, under products/paas/shopware/guides, detailing how to enable Opensearch in Shopware by updating the application.yaml file and committing the changes.
• Included instructions for reindexing the application after Opensearch is enabled, using the sw-paas exec command and the Shopware console.

[Copilot]

Pull request overview

Adds a new Shopware PaaS Native documentation guide explaining how to enable and use OpenSearch/OpenSearch (service: opensearch) for a Shopware application.

Changes:

• Added a new guide describing how to enable the opensearch service in application.yaml.
• Documented the required post-enablement reindexing step using sw-paas exec and Shopware console commands.

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[Copilot]

The product name is typically written as “OpenSearch” (capital S). Consider updating the nav title (and other occurrences in this guide) to use the official capitalization to avoid inconsistent branding/searchability.