Create RBAC overview page for SDK

• Create a high level overview page (not SDK language specific) that describes RBAC usage via SDK.  
• This page MUST mention three critical items:
  • With older SDKs, to enable authentication, create a user with the same name as the bucket name.
  • Couchbase server 5.0 does not allow passwordless authentication, other than via a bucket upgraded from an earlier version.  So, older code that would create a bucket without a password (say in unit tests), will need to be updated.  Ideally this is best done with the latest SDKs that have support for user creation, etc.
  • Upgrades of production environments from earlier versions to 5.0 should not see any impact, except at the time of new bucket creation.
• some new authentication examples can be shown
• mention that the "start using SDK" language specific pages for SDKs with Couchbase Server 5.0 show detailed examples.

Our overall goal for this is a UX that matches the following goals:

A) When a user tries an old piece of software (SDK, Sync Gateway, connector, etc.) against 5.0 and it does not work because of {blank password|no user|missing permission} there is a document at a URL that {support|forums|IM} conversations can point to educate the user on how to fix it.  This will be used A LOT over the next couple years.

B) The most recent old software docs are updated with a callout box pointing to what’s in A.

This ticket, in effect, tracks A.  Along with this, I would argue you'll need to get an evergreen URL redirect created.  Something like "http://developer.couchbase.com/documentation/auth-faq" which would be updated to point to the latest overview in 5.0, then 5.1, etc.