Create initial Cells development guide

What does this MR do?

Create initial Cells development guide

We need this so that other teams can start to fix all workflows.

Most of the material is already present in the multiple databases guide, so we just add a specific page for Cells

I anticipate we may add additional sections later for other workstreams like the Routing Layer.

Related issues

Related issue: #388282 (closed)

Author's checklist

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• ~"frontend"
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• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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