Add percentile example

[ghost]

I have made an attempt to better understand percentiles in Atlas and hopefully address this issue as well: #718.

Given the following request latencies:

And applying the example query yields the following result:
name,requestLatency,:eq,(,25,50,90,),:percentiles

Unfortunately, from the generated docs, the input graph looks as follows, due to aggregation:

Obviously having percentiles generate from a single input does not make sense, therefore ideally it would be possible to generate the example input using :all, but I didn't pursue this route any further.

Please let me know if I can enhance the example in any way!

[ghost]

As a follow up, I am happy to add some notes on my findings as a separate PR (issue #347).

[brharrington]

Thanks for the contribution. I'm out of office until next week, but will look into the example then.

[brharrington]

There is a work in progress to migrate the documentation to: https://github.com/Netflix/atlas-docs. This should make it easier to contribute to as well as give some more control over some of the aspects. The auto-generated examples are convenient in some ways, but often have restrictions that make them less than ideal for conveying the information.