Add docs for runner fleet dashboard early adopters program

What does this MR do and why?

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

We want to test the new Runner Fleet dashboard with a few selected self-managed clients. This MR provides the documentation on how to setup Clickhouse database and enable necessary feature flags for such tests.

This page is intentionally placed in our development section of the documentation and not the user one. The idea is to emphasize that using Clickhouse is still experimental, and we don't yet fully support it.

I also plan to record a video walking through this.

Ideally in the future this documentation should be broken into 3 separate pages:

1. Normal dashboard documentation which doesn't require Clickhouse
2. Clickhouse setup docs
3. Dashboard setup docs

But I on purpose budled it up toghether into a single page for now.

Review app: http://main-ee-135227.docs.gitlab-review.app/ee/development/runner_fleet_dashboard.html

Screenshots or screen recordings

I'm still working on this.

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