Add deployment flow diagram to PREP docs

and simplest way for any new feature or changeset to reach GA status across all deployment platforms.

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### GitLab Deployments

The following diagram illustrates how GitLab features flow through different deployment targets. PREP facilitates this process at each step.

```mermaid

flowchart TD

    A[Architecture Design Document] --> B(Dev)

    B --> E

    B --> L

    subgraph GitLab.com

      E(staging.gitlab.com) --> G[gitlab.com]

      L(cellsdev) --> M[cellsprod]

    end

    G -->|Gather & Tag| K(Publish)

    K --> H2

    subgraph Self-Managed-Deploy

      N(Deploy)

    end

    subgraph Self-Managed-Ops

      O(Ops)

    end

    subgraph Self-Managed-Dev

      P(Dev)

    end

    K --> N

    K --> O

    K --> P

    subgraph Self-Managed

      H2(Dedicated sandbox) --> I2(UAT) --> J2[Dedicated preprod / Dedicated production]

    end

```

## When is the best time to engage with PREP?

We strongly recommend engaging with PREP as early as possible for any feature that introduces or makes significant changes to _infrastructure or core components_. Here are a