Document JSON Schema inputs on the manual trigger

## Summary

Workflow input docs still show the legacy workflow-level array form. Inputs now live on the **manual trigger** and are JSON Schema (including `$ref` to built-in Kibana definitions). That is not documented.

**Requested by:** Nastasha Solomon in [#one-workflow](https://elastic.slack.com/archives/C08TKN4P51V/p1786974122189449?thread_ts=1786515935.250359) so the docs match the 9.5 inputs move and versioning. JSON Schema / `$ref` also came up in [this thread](https://elastic.slack.com/archives/C08U04SUN49/p1787161420415709) ([built-in types discussion](https://elastic.slack.com/archives/C08U04SUN49/p1778610412071949)).

## Current State

- [Manual triggers](https://www.elastic.co/docs/explore-analyze/workflows/triggers/manual-triggers) still says inputs are defined at the workflow level and only shows the array form
- [Anatomy](https://www.elastic.co/docs/explore-analyze/workflows/authoring-techniques/anatomy) notes 9.5+ trigger placement, but examples are still the array form; JSON Schema is a one-liner
- [Cheat sheet](https://www.elastic.co/docs/explore-analyze/workflows/reference/cheat-sheet) still shows top-level `inputs: [ ... ]`
- No docs for JSON Schema inputs or `$ref: '#/kibana/definitions/'`

## Proposed Solution

Update the pages above (and any other hits) for 9.5+ / serverless:

- Put `inputs` on the `manual` trigger, not the workflow root
- Document JSON Schema as the current form; keep the array form as legacy / 9.4
- Document `$ref: '#/kibana/definitions/'` (e.g. `alertingV2NotificationGroup`)
- Use version tabs where the syntax changed

Example of the current shape:

```yaml
triggers:
- type: manual
inputs:
type: object
properties:
environment:
type: string
default: staging
notificationGroup:
$ref: '#/kibana/definitions/alertingV2NotificationGroup'
required: [environment]
```

## Out of Scope (for now)

- Event typing on manual triggers ([security-team#19006](https://github.com/elastic/security-team/issues/19006))
- Product work for the inputs move itself

Start with the Manual triggers, Anatomy, and Cheat sheet pages named in the issue, then search the documentation for other top-level inputs examples. Update the 9.5+ and serverless guidance to show manual-trigger JSON Schema inputs and the Kibana $ref form, while labeling the array syntax as legacy or 9.4. Done means the versioned examples and explanatory text agree across all affected pages.

Written by the indexing model from the issue text.