[DOCS] Rule preview enhancements: side panel, exceptions, overrides, prebuilt rules

[joepeeples]

Description

PR elastic/kibana#140221 adds several enhancements to detection rule previews:

• Rule preview is now a persistent side panel/flyout during rule creation/editing
• Exceptions are included in preview
• Field overrides are included in preview
• Rule preview now available for prebuilt rules

Notes

• This replaces the "advanced preview" introduced in 8.4. With these enhancements, basically every preview is advanced now.