docs: Add enumSuggestedValues to input schema spec. (#2111)

Document the new `enumSuggestedValues` property in the input schema
introduced by apify/apify-shared-js#569

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---

> [!NOTE]
> Documents `enumSuggestedValues` for `select` editors (string and
array) with examples, and updates tables/guidance to distinguish strict
`enum` validation from suggested values.
> 
> - **Input schema docs**
(`sources/platform/actors/development/actor_definition/input_schema/specification.md`):
>   - **String type**:
> - Add `enumSuggestedValues` option to table with description and
clarify `enumTitles` applies to `enum` and `enumSuggestedValues`.
>     - Note that `enum` enforces strict validation.
>   - **Select editor (string)**:
> - Add separate examples for strict `enum` vs suggested
`enumSuggestedValues`, including UI screenshots.
>   - **Array type (multiselect)**:
>     - Introduce "Select editor for arrays" section.
> - Add strict multiselect using `enum` and suggested multiselect using
`enumSuggestedValues` examples with screenshots.
> - Clarify defining options via `items` with
`enum`/`enumSuggestedValues` and `enumTitles`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
6f408c8. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: mfori