[DOCS] Add Webhook connector to case and connector docs

[nastasha-solomon]

Description

A new Webhook connector has been developed for cases created in Stack Management, Observability, and Security. Fields that need to be added are doc'd at elastic/kibana#137726.

Related issues:

• [Security Solution] Need generic (webhook) connector for Cases kibana#124687
• [Security Solution] Webhook - Case Management Connector kibana#131762

Required doc updates

Will need to update the connectors list and add relevant reference material in the following topics:

• Configure external connections (Update connector list and reference topic Steph creates for the Kibana docs)
• Cases(Update connector list.)
• Connector API docs: (?)
  • Create or update an external incident
  • Create connector

Notes:

• This connector will be doc'd in the 8.4 Kibana release notes under the newly created "Connectors" section. Can point to those release notes (or the actual section) from the new features section in the 8.4 Security release notes.
• Note about the connector docs:
  • The docs about closing cases after pushing updates might be outdated. Check if can remove it.

[nastasha-solomon]

Steph is creating the reference material that'll go into the Kibana docs. I'll need to grab that content when updating the "Configure external connections" topic.

[nastasha-solomon]

Merged #2221.