Improve module documentation: fix README inaccuracies and add dedicated docs for configuration, developer guide, supported entities, and entity_clone_extras

The current README.md contains two factual inaccuracies that mislead site builders and developers:

• "The module has no menu or modifiable settings. There is no configuration." — The module ships with a full admin settings UI at /admin/config/system/entity-clone with two sub-pages.
• "This module requires no modules outside of Drupal core." — The module bundles the entity_clone_extras sub-module and declares several optional test dependencies.

This issue fixes those errors and introduces a structured docs/ folder to cover the module's full feature set, which today has no written documentation at all.

Changes included:

• README.md: corrected requirements and configuration sections; added links to sub-modules and the new docs/ pages.
• docs/configuration.md: full reference for both admin pages, all global settings, the clone form settings table, and the complete permissions model (static + dynamic).
• docs/developer.md: handler architecture overview, writing custom content/config handlers, the $already_cloned circular reference guard, PRE_CLONE/POST_CLONE event subscribers, hook_entity_clone_settings_alter, OOP hook pattern, and known pitfalls.
• docs/supported-entities.md: handler-to-entity-type mapping, behaviour notes for all specialised handlers, and a guide for adding support for custom entity types.
• modules/entity_clone_extras/README.md: new dedicated README covering purpose, installation, per-bundle permission naming convention, and how it differs from the default access model.