Break down ES guide into two separate guides for small and big instances

The approaches we use for small and big instances are completely different and I think It makes a lot of sense to break them down to not confuse people.

@nick.thomas I think we should do it for 9.0. @smcgivern What do you think? do we have a capacity?

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How long would it take to do this?

I think something about 3hrs.

Yep, I think we can do this under the general 'ES improvements' umbrella, nice suggestion!

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@vsizov would you like more information on how to set this up for multiple instances?

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@SeanPackham I think it's covered by ES official docs, from our side the user only need to specify all the nodes in the settings. So, not sure what information else you mean?!

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mentioned in issue #2307

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assigned to @vsizov

mentioned in merge request !2401 (merged)

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2401

closed via merge request !2401 (merged)