Document ClickHouse migrations for Runner Fleet Dashboard

What does this MR do and why?

With Add basic migration support for ClickHouse (!136103 - merged) users no longer need to create schema manually, just run the rake task.

Related to Write documentation for using CI Analytics with... (#424500 - closed)

How to set up and validate locally

Just read the docs =)

I will record the walkthrough of the whole process later.

MR acceptance checklist

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