Rename authentication.yaml to auth.yaml (#796)

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# [Issue-793](#793)


## What changed?
Making left navigation consistent

## Release notes draft
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language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* 

## Anything else?
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ping {names}