Communicate upcoming Drupal core linking enhancement

Problem/Motivation

With #3317769: Add support for linking to entities in CKEditor 5 likely arriving in core in the near future, it will be useful to communicate, in advance, to developers using Linkit that Drupal core will provide similar functionality.

The README and project page and the documentation guide should include the following information:

- Announcing that Drupal core will provide a similar functionality in the future
- Enumerating what functionality Linkit will have that won't initially be in core

This change can constitute the final code change before a full release of Linkit 6.0.0 and 6.1.0, which have been planned in #3345480: LinkIt Release Roadmap and Issue Prioritization to arrive in July 2023.

Providing a full release should also resolve the following issues with updates:
- #3373126: Update core 10.0.x to 10.1.x not possible
- #3363992: Update module erroneously recommends conflicting update to 6.1.x for sites on Drupal 10.0.x or lower