docs(32-01): form extensions in admin-controllers, cabana and pact READMEs

- admin-controllers: placement, values, Load/Save, permissions,
  widgets, partials and boot errors, with the ExtrasPlugin snippet
- Example_formExtension compiles an owner form with the extension
- coming-from-wintercms maps extendFormFields, addFields, addTabFields
  and removeField to the form extension
- cabana README feature bullet and Activate row

docs/setup/coming-from-wintercms.md

@@ -26,6 +26,7 @@ Each row names the WinterCMS concept, the SummerCMS identifiers that replace it,
 | Laravel collections: `map`, `filter`, `reduce`, `keyBy`, `groupBy`, `unique`, `partition`, `first` and `pluck` | Generic functions on plain slices and maps: `sieve.Map`, `sieve.TryMap`, `sieve.Filter`, `sieve.Reduce`, `sieve.KeyBy`, `sieve.GroupBy`, `sieve.Uniq`, `sieve.Partition` and `sieve.Find`. Pluck is `sieve.Map` with a field accessor, unique by key is `sieve.UniqBy`, `chunk` is the standard `slices.Chunk`, and sets are built with `sieve.SetOf` | [sieve](../../modules/sieve/README.md) |
 | `fields.yaml` and `columns.yaml` | The same YAML, embedded in the plugin through `pact.AdminAssets` and compiled at boot into a `cabana.CompiledController` | [Forms](../backend/forms.md), [Lists and filters](../backend/lists-and-filters.md), [cabana](../../modules/cabana/README.md) |
 | Backend controllers with the Form, List and Relation behaviours | A `pact.AdminController` returned from `pact.HasAdminControllers`; the generic admin API replaces the behaviours, and hooks such as `pact.FormBeforeCreate` and `pact.ListExtendQuery` replace behaviour overrides | [Admin controllers](../backend/admin-controllers.md), [cabana](../../modules/cabana/README.md) |
+| `extendFormFields` (the `backend.form.extendFields` event) with `addFields`, `addTabFields` and `removeField` | A form extension: `pact.HasAdminActionExtensions` returning a `pact.AdminActionExtension` whose `Form` is a `pact.AdminFormExtension`. Its fields document places fields `before:` or `after:` an owner field and declares new tabs under `tabs:` (`addFields`, `addTabFields`, with real placement instead of the end of the form), `Hide` replaces `removeField`, and `Load` and `Save` keep the values in the plugin's own tables inside the record's save transaction | [Admin controllers](../backend/admin-controllers.md#adding-fields-to-another-controllers-form), [pact](../../modules/pact/README.md) |
 | The Reorder behaviour and `config_reorder.yaml` | A `config_reorder.yaml` next to `config_list.yaml` and the `reorder` toolbar button: a drag-and-drop tree, or a flat list for records without a parent column; `pact.ReorderAfterSave` runs after a reorder save | [Lists and filters](../backend/lists-and-filters.md#tree-reorder), [cabana](../../modules/cabana/README.md) |
 | Form widgets and `registerFormWidgets` | A plugin field type: `pact.HasFormFieldTypes` returning `pact.FormFieldType` values, usable as `type:` in any controller's `fields.yaml`; a one-off element with a server action is a `type: widget` field | [Plugin field types](../backend/field-types.md), [Partials and widgets](../backend/partials-and-widgets.md#form-widgets) |
 | HTML cleaning of rich editor content | Every `richeditor` value is sanitized on save by `seaglass.Sanitize`, an allowlist policy; plugin code calls `seaglass.SanitizeReport` to learn what was removed | [Rich text](../backend/forms.md#rich-text), [seaglass](../../modules/seaglass/README.md) |