[Doc] Add documentation for headless enterprise features in ra-core documentation

[djhi]

Problem

We now have headless support for some React Admin Enterprise features. It's time to mention them in the ra-core documentation.

How To Test

• make doc-headless

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature

Screenshots

[Copilot]

Pull Request Overview

This PR adds documentation for headless React Admin Enterprise features to the ra-core documentation. It introduces comprehensive documentation for enterprise features like real-time subscriptions, record locking, and role-based permissions in a headless context.

• Adds documentation for 20+ enterprise hooks and utilities including real-time subscriptions, record locking, and permission management
• Updates the documentation site configuration to include a new "Realtime" section with enterprise-specific styling
• Adds CSS styling for enterprise features with premium badges and theme-aware icons

Reviewed Changes

Copilot reviewed 21 out of 23 changed files in this pull request and generated 5 comments.

File	Description
docs_headless/src/styles/global.css	Adds CSS styling for enterprise links with premium badges and theme-aware icons
docs_headless/astro.config.mjs	Adds new "Realtime" section and enterprise entry helper function for documentation navigation
Multiple .md files	Comprehensive documentation for enterprise hooks covering real-time subscriptions, record locking, permissions, and live data fetching

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