ESK: add Using Secrets tutorial

[bharvey88]

Summary

Adds a new Tutorials subsection under ESPHome Starter Kit, with a first page covering secrets.yaml beyond Wi-Fi. The Getting Started page already introduces secrets.yaml for Wi-Fi only; this tutorial picks up from there and covers the other values most users end up storing.

The page walks through:

• What secrets.yaml is and why to use it
• Opening it in ESPHome Device Builder (reuses existing esphome-device-builder-click-secrets.gif and esphome-device-builder-secrets.png assets)
• !secret syntax
• Sectioned entries with the secrets.yaml line and the device YAML line side by side for: Wi-Fi, fallback hotspot password, Home Assistant API encryption key, OTA password, web_server: auth, and MQTT broker/username/password
• A complete example secrets.yaml
• Good practice notes (don't share, keep a backup, rotate in one place)

Nav now reads:

ESPHome Starter Kit:
  - First Steps
  - Getting Started
  - Tutorials:
      - Using Secrets

URL slug: /tutorials/using-secrets/.

Test plan

• Page renders in CloudCannon preview on dev
• Sidebar shows the new Tutorials → Using Secrets entry under ESPHome Starter Kit
• Both image assets resolve
• Link from this page back to Getting Started works

Summary by CodeRabbit

• Documentation
  • New tutorial documentation on securely storing and managing Wi-Fi credentials, API keys, and other sensitive configuration data in ESPHome projects. Includes practical examples and best practices for handling secret values.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: e5a0284b-d6c0-410b-9f60-f7a8531c7325

📥 Commits

Reviewing files that changed from the base of the PR and between d3ec15b and cf5e6d7.

📒 Files selected for processing (2)

• docs/products/ESPHome-Starter-Kit/tutorials/using-secrets.md
• mkdocs.yml

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Walkthrough

This PR adds a new tutorial documenting ESPHome Device Builder's secrets.yaml file, explaining its purpose for secure credential storage, the !secret reference syntax, practical examples for common credentials across multiple features, and best practices. The tutorial is integrated into the site navigation structure.

Changes

ESPHome Secrets Usage Tutorial

Layer / File(s)	Summary
Concept and accessor explanation docs/products/ESPHome-Starter-Kit/tutorials/using-secrets.md	Introduction to secrets.yaml purpose and the !secret <key> syntax, with documentation of build-failure behavior when keys are missing.
Practical per-feature examples docs/products/ESPHome-Starter-Kit/tutorials/using-secrets.md	Detailed structured guidance on storing Wi‑Fi credentials, fallback hotspot password, Home Assistant API encryption key, OTA password, web server credentials, and MQTT settings, with corresponding device YAML reference patterns for each.
Complete example and best practices docs/products/ESPHome-Starter-Kit/tutorials/using-secrets.md	Combined secrets.yaml reference example covering all credential types, clarification that not every device needs every entry, security best-practice notes, and recommended next steps including re-flashing and resource links.
Navigation and discoverability mkdocs.yml	Updated site navigation to add a new Tutorials subsection under ESPHome Starter Kit, linking to the "Using Secrets" tutorial page.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🐰 A guide to secrets so fair,
Where credentials safely rest with care,
No Wi‑Fi keys in configs bare,
Just !secret everywhere,
And next steps bloom beyond compare! 🌿

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/esk-using-secrets

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