[SIEM] Detections Markdown-Capable Field Design Mockups

[MichaelMarcialis]

Craft design mockups that support the addition of a new, Markdown-capable "documentation" field within the advanced settings of a detection rule's about step. Also account for the impact this addition will have on the details page for a rule utilizing this field.

Mockups
Clickable Prototype

Originating Issue

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)