docs: Add Spring Boot (REST) data provider to DataProviderList.md

[femrek]

Problem

As I mentioned this issue, current data provider for Spring does not includes backend specifications. And I have developed a Frontend+Backend library to solve this end to end.

This pr adds my repo to data provider list page. This pr adds a new line under the existing Spring Boot line but you may think about replacing due to inactivity.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why): just a doc update
• The PR includes one or several stories (if not possible, describe why): just a doc update
• The documentation is up to date

[Copilot]

Pull request overview

This PR updates the React-Admin documentation to reference an additional Spring Boot-based integration, helping users discover a Spring-compatible option that aligns with the REST data provider style.

Changes:

• Adds femrek/ra-spring-json-server as a new Spring Boot (REST) option in the DataProviderList.md documentation.

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