[DOCS] Improve documentation for the bulk actions API

[banderror]

Epic: elastic/kibana#200310
Based on the feedback from @Tibo669 in #2985

Summary

Let's improve the docs for the Bulk rule actions API endpoints.

Current issues with this page:

• It documents several different endpoints and contains a lot of text that is hard to navigate.
• Despite the size, it's not super strict and clear in terms of request and response schemas, for example, rule action parameters.
• Despite the size, it lacks a lot of examples for every bulk action we support, especially bulk editing.

Criteria:

• Each bulk endpoint should be documented on its own separate page: _bulk_create, _bulk_delete, PUT _bulk_update, PATCH _bulk_update, _bulk_action.
• The page for the _bulk_action endpoint should contain a common description, request/response schema, and examples of usage of this endpoint. E.g. docs on the dry-run mode.
• Each concrete bulk action should be documented on its own sub-page under _bulk_action: delete, disable, enable, export, duplicate, and edit.
• All schemas should be documented in detail and with examples. For example, for each connector type we support in bulk editing rule actions, we should have a separate section documenting the request schema and examples of usage.