Add new copy to AGCDN Blue/Green Deployment section

[ccharlton]

Re: Blue/Green Deployment section of AGCDN docs. We got more content for it. Yay!

Priority: Medium

Issue Description

• The content for Blue/Green Deployment support needs to be expanded.

How will this impact users?

• Improves documentation.
• Enables self-service for customers.
• Helps reduce draw on support to answer basic info.
• Provides a consistent place for product, marketing, sales, and support to reference in customer communications.

Suggested Resolution

• Add content to public docs taken from internal guide titled 'Guide to using AGCDN for Blue/Green Deployments on Pantheon'. (document link redacted but available upon request)

[rachelwhitton]

@ccharlton could you open a PR for this when convenient?

[ccharlton]

@rachelwhitton I did not want to upload the internal content without clearance first. Ping me if there's more coordination needed for that. Please advise.