Update REST API documentation and add Relation Modal documentation

[dev-docs-github-app-dev]

This pull request includes two main changes:

1. Update to the REST API documentation in docusaurus/docs/cms/api/rest.md:

   • Improve the description of the REST API reference
   • Standardize the formatting and organization of the documentation

2. Addition of a new documentation page for the Relation Modal feature in docusaurus/docs/cms/features/relation-modal.md:

   • Provide an overview of the Relation Modal feature in Strapi 5
   • Explain how to use the Relation Modal, including opening, navigating, editing, and creating related content
   • Discuss configuration options and best practices for the Relation Modal
   • Include technical details about the implementation of the Relation Modal

This PR is generated from the issue team-dev-docs/strapi#11