docs: add Getting Started guide — Docker deployment, first login, and data entry walkthrough

[jeandavidt]

Goal

Create a beginner-friendly Getting Started guide that takes a user from zero to their first recorded observation. This is the single most important onboarding document for new adopters.

Sections to Cover

1. Deploy with Docker

• Prerequisites (Docker, .env.docker)
• docker-compose up sequence (db → db-init → api → app)
• Verify: API docs at localhost:8000/docs, web app at localhost:8501

2. First Login

• How to access the web interface
• Initial account / auth setup (if applicable)

3. Understand the Data Model (conceptual intro)

Explain the core concepts in plain language, connecting them to real WRRF operations:

Concept	What it means
Site	A physical treatment plant or monitoring location
Equipment	A physical instrument (e.g., a DO probe, flow meter)
SignalPort	A logical output of an instrument (e.g., "DO channel", "temperature output") — one instrument can have many signal ports
Channel	A time series stream — links a SignalPort to a Parameter + Unit. This is where measurements land
SCADA tag	The string identifier used by a SCADA/data acquisition system to reference a signal port
Observation	A raw reading event tied to a Channel with a timestamp
Value	The numerical payload of an Observation (scalar, vector, matrix, or image)

4. Add Your First Data Manually

• Create a Site
• Register Equipment and an EquipmentModel
• Create a SignalPort and bind it to a Channel
• Record a manual observation via the interface or API

5. Configure Automated Data Ingestion

• Tag-mode ingestion: map SCADA tags to SignalPorts so incoming data self-routes
• Tagless-mode ingestion: direct-connect stations that bypass tag resolution
• Link to architecture guide for full detail

6. Verify Data is Flowing

• How to query the latest values for a Channel
• What the Explore page in the UI shows

Notes

• The README already has a brief Docker quick-start — this guide should expand on it significantly
• Should be added to the MkDocs nav as a top-level "Getting Started" section
• Cross-link to architecture guides for deeper explanations

Acceptance Criteria

• Guide is reachable from the docs home page
• A user with no prior knowledge can follow it end-to-end
• All key concepts (SignalPort, Channel, SCADA tag, Observation, Value) are defined
• Docker deployment steps are tested and accurate for the current docker-compose.yml