chore(docs): add frontend migration guides (#5009)

* chore(docs): add frontend migration guides

This change adds a couple migration guides to help plugin developers and
RHDH operators better understand the changes required to adopt the new
frontend system once it's made the default in RHDH.

Assisted-By: Cursor Desktop

* docs: add gotchas, API changes, drawer blueprint, and reference PRs

- Fix drawer section: AppDrawerContentBlueprint exists in RHDH (not
  "pending NFS design")
- Add translation module pluginId: 'app' requirement
- Add API breaking changes (param renames, makeWithOverrides deprecation)
- Add 3 common gotchas (defineParams wrapper, double headers, default export)
- Add reference migration PRs table (8 rhdh-plugins PRs)
- Add dev app setup for Phase 3 validation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Rohit Rai <rohitkrai03@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

docs/dynamic-plugins/frontend-plugin-wiring.md

@@ -1,5 +1,10 @@
 # Frontend Plugin Wiring
 
+> **Legacy configuration reference.** This document describes RHDH dynamic plugin frontend wiring for the **current** app shell. If you are moving to the Backstage new frontend system, use:
+>
+> - **Operators / platform admins** (YAML customization only) → [Migrating Config to the New Frontend System](migrating-config-to-new-frontend-system.md)
+> - **Plugin authors** (plugin code and `/alpha` exports) → [Migrating Plugins to the New Frontend System](migrating-plugins-to-new-frontend-system.md)
+
 Compared to the backend plugins, where mount points are defined in code and consumed by the backend plugin manager, frontend plugins require additional configuration in the `app-config.yaml`. A plugin missing this configuration will not be loaded into the application and will not be displayed.
 
 Similarly to traditional Backstage instances, there are various kinds of functionality a dynamic frontend plugin can offer:

docs/dynamic-plugins/index.md

@@ -21,6 +21,10 @@ More details about publishing dynamic plugins is in the [Packaging Dynamic Plugi
 
 [Frontend Plugin Wiring](frontend-plugin-wiring.md)
 
+[Migrating Plugins to the New Frontend System](migrating-plugins-to-new-frontend-system.md)
+
+[Migrating Config to the New Frontend System](migrating-config-to-new-frontend-system.md)
+
 [Local configuration (for development only)](local.md)
 
 [Version Compatibility Matrix](versions.md)