Update blue/green deployment docs

[abrennan89]

This issue is to update the docs about using the blue/green traffic routing pattern.
Existing docs are here: https://knative.dev/development/serving/samples/blue-green-deployment/

Tasks required:

• Move these docs from the obscurity of serving/samples to their own section in the docs under dev guide.
• Add alternative kn CLI commands as well as cleaning up kubectl/YAML docs.

May require some additional info from eng re commands to add.

[evankanderson]

Here's a CLI command to split traffic:

kn service update example --traffic green=80 --traffic blue=20

To assign 80% of traffic to green and 20% to blue revisions of the "example" Service. Note that the actual Revision names will probably be more like "example-green" and "example-blue" -- you'll probably want to use the user-selected revision name option in the YAML/the --revision-name flag to kn service create and update commands.

/remove-triage needs-eng-input