feat: add architecture overview page with Mermaid diagram

[lbajsarowicz]

Summary

• Add new Architecture Overview page to the documentation with an interactive Mermaid diagram
• Add sphinxcontrib-mermaid extension for rendering Mermaid diagrams during Sphinx build
• Add page to the "Getting Started" toctree (between "Installing" and "Services")
• Exclude .venv from Sphinx build in .gitignore and conf.py

The architecture page documents Warden's three service layers with:

• A visual diagram (see preview below)
• Service tables with URLs and toggle flags
• Descriptions of all optional services (Blackfire, SPX, Selenium, Allure, etc.)

Architecture Diagram Preview

flowchart TB
    classDef default fill:#ffffff,stroke:#3f51b5,stroke-width:1.5px,color:#000000,font-weight:bold,font-size:12px;

    subgraph DevSys ["Developer's System"]
        direction TB

        subgraph Host ["Host Services"]
            direction LR
            h1["Warden CLI"] ~~~ h2["Docker Engine"] ~~~ h3["Mutagen File Sync"] ~~~ h4["Trusted SSL Root CA"]
        end

        subgraph Global ["Global Docker Services — warden svc up"]
            direction LR
            g1["Traefik<br>Reverse Proxy + SSL"] ~~~ g2["DNSmasq"] ~~~ g3["Mailpit<br>Email Catcher"] ~~~ g4["SSH Tunnel"] ~~~ g5["phpMyAdmin"] ~~~ g6["Portainer"]
        end

        subgraph Project ["Per-Project Docker Services — warden env up"]
            direction LR
            p1["Nginx"] ~~~ p2["PHP-FPM"] ~~~ p3["MariaDB / MySQL"] ~~~ p4["Redis / Valkey"] ~~~ p5["Elasticsearch /<br>OpenSearch"] ~~~ p6["RabbitMQ"] ~~~ p7["Varnish"]
        end

        Host ~~~ Global ~~~ Project
    end

    style DevSys fill:#f5f5f5,stroke:#666666,stroke-width:1px,rx:8,ry:8
    style Host fill:transparent,stroke:none
    style Global fill:#fff3e0,stroke:#FF9800,stroke-width:1px,rx:8,ry:8
    style Project fill:#e8f5e9,stroke:#4CAF50,stroke-width:1px,stroke-dasharray: 5 5,rx:8,ry:8

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Test plan

• Run pip install -r requirements.txt && make html — verify architecture.html renders the Mermaid diagram
• Check the diagram renders correctly on the Architecture Overview page
• Verify the toctree includes the new page between "Installing" and "Services"

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