[8.15] Add case templates (backport #5565)

[mergify]

This PR updates https://www.elastic.co/guide/en/security/current/cases-open-manage.html to include steps related custom fields and templates.

It also renames and restructures https://www.elastic.co/guide/en/observability/current/cases-external-connectors.html so that it more clearly pertains to all of the case settings, not just the connector-related ones. To that end, it adds a new section for the 8.15 case templates feature on the new page. In a subsequent PR, I will move the custom field information there as well. I am doing it separately since that custom fields feature needs significant updates: it is going GA and needs screenshot refreshes.

Preview

• https://security-docs_bk_5565.docs-preview.app.elstc.co/guide/en/security/master/cases-open-manage.html (added steps in case creation related to templates and custom fields)
• https://security-docs_bk_5565.docs-preview.app.elstc.co/guide/en/security/master/cases-manage-settings.html (page renamed and reorganized, new template section added, reduced the number of images)

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This is an automatic backport of pull request #5565 done by [Mergify](https://mergify.com).

[github-actions]

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