Restructure Overview section of the Admin guide v1.12.x and v1.13.x-SNAPSHOT

[kiran1287]

• Rewrite the Admin Guide intro for clarity and conciseness across all three versions
• Replace a dense run-on sentence with a bulleted list of admin capabilities (Roles & Policies, Services & Notifications, Custom Properties & Data Insights, Users & Teams)
• Update "three quick and easy steps" to plain "these three steps"

File need changes

• v1.12.x/how-to-guides/admin-guide.mdx
• v1.13.x-SNAPSHOT/how-to-guides/admin-guide.mdx