📖 Update migration docs to reflect current tooling and options

Rewrites the migration documentation to focus on current migration paths:
- AutoUpdate plugin for automated GitHub Actions-based updates
- Alpha update command for local migration workflows
- Alpha generate for manual re-scaffolding
- Traditional manual migration for legacy projects

Removes outdated information and clarifies when each approach should be used.
Adds guidance on the PROJECT file and its role in automation.

Author: camilamacedo86

docs/book/src/SUMMARY.md

@@ -49,22 +49,6 @@
 
 - [Migrations](./migrations.md)
 
-  - [Legacy (before <= v3.0.0)](./migration/legacy.md)
-    - [Kubebuilder v1 vs v2](migration/legacy/v1vsv2.md)
-
-      - [Migration Guide](./migration/legacy/migration_guide_v1tov2.md)
-
-    - [Kubebuilder v2 vs v3](migration/legacy/v2vsv3.md)
-
-      - [Migration Guide](migration/legacy/migration_guide_v2tov3.md)
-      - [Migration by updating the files](migration/legacy/manually_migration_guide_v2_v3.md)
-  - [From v3.0.0 with plugins](./migration/v3-plugins.md)
-    - [go/v3 vs go/v4](migration/v3vsv4.md)
-
-      - [Migration Guide](migration/migration_guide_gov3_to_gov4.md)
-      - [Migration by updating the files](migration/manually_migration_guide_gov3_to_gov4.md)
-  - [Single Group to Multi-Group](./migration/multi-group.md)
-
 - [Alpha Commands](./reference/alpha_commands.md)
 
   - [alpha generate](./reference/commands/alpha_generate.md)