fix: add missing service README.md files with basic description

[schweigisito]

Description

Add missing README.md files, including some basic servcie description, to some of the affected services

Motivation and Context

docs build pipeline fails because of missing files, therefore missing links within the docs files.

How Has This Been Tested?

Not yet, will be tested with a next 'markdown-docs-generatoranddocs` build run

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation added

[Copilot]

Pull Request Overview

This PR adds missing README.md files to several OpenCloud services to resolve documentation build pipeline failures caused by broken links. Each README provides a basic description of the service's purpose, integration points, and scalability characteristics.

• Adds comprehensive README.md files for six services (users, storage-shares, storage-publiclink, sharing, groups, app-provider)
• Documents CS3 API implementations and backend integrations
• Includes service-specific configuration and scalability information

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
services/users/README.md	Documents the CS3 Users API service for managing user information and authentication
services/storage-shares/README.md	Documents the storage backend for user and group shares with virtual shares folder
services/storage-publiclink/README.md	Documents the storage backend for public link shares and anonymous access
services/sharing/README.md	Documents the CS3 Sharing API for managing user and public link shares
services/groups/README.md	Documents the CS3 Groups API service for managing groups and memberships
services/app-provider/README.md	Documents the CS3 App Provider API for managing file-opening applications

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[butonic]

nice, but some things not quite accurate