docs: add missing title property to webhook API schemas

[marcel-rbro]

Summary

• Adds the missing title property to WebhookCreate, WebhookUpdate, and Webhook OpenAPI schemas
• The Apify API supports a title field on webhooks, but it was not documented in the OpenAPI spec

Closes #2035

Test plan

• Verify the OpenAPI spec still validates correctly
• Check that the webhook create/update/get endpoints show the title property in rendered API docs

Generated with Claude Code

[apify-service-account]

Preview for this PR was built for commit f242701 and is ready at https://pr-2249.preview.docs.apify.com!

[marcel-rbro]

Review: LGTM

The change correctly adds the missing title property to all three relevant webhook OpenAPI schemas.

What looks good

1. Pattern consistency: The title property definition (type: [string, "null"] with examples) exactly matches the pattern used by the adjacent description property in all three files. Good adherence to existing conventions.

2. Cross-schema consistency: The definition is identical across Webhook.yaml, WebhookCreate.yaml, and WebhookUpdate.yaml, which is correct since they should all support the same title field.

3. Logical placement: title is placed immediately before description in each schema, which is a natural grouping.

4. Example value: Actor run succeeded webhook is appropriate - it matches the ACTOR.RUN.SUCCEEDED event type used in other examples in the same files, making the schema self-consistent.

5. All CI checks pass: Lint, spec validation, bundled spec build, docs build, and spell check all green.

Considered but not an issue

I checked whether WebhookShort.yaml (used by the list webhooks endpoint via ListOfWebhooks.yaml) also needs title. It does not - WebhookShort already omits description, payloadTemplate, and headersTemplate, so omitting title is consistent with the "short" representation pattern.

No changes requested. This is a clean, minimal fix that addresses issue #2035.