Add a user guide for Session Persistence

[salonichf5]

Add a user guide for session persistence in OSS and Plus

[sjberman]

Some extra things to consider here:

• UpstreamSettingsPolicy guide should mention that loadBalancingMethod is supported, maybe in the top list where we have the directives that are supported.
• Session Persistence guide should have a callout that it's an experimental API that is subject to change.