Add security best practices documentation for users

[bdraco]

Description:

This PR adds security best practices documentation for ESPHome users. The document provides practical guidance on securing ESPHome devices and deployments.

Note merges to next because it talks about features only in next.

Related issue (if applicable): N/A - New documentation

Pull request in esphome with YAML changes (if applicable): esphome/esphome#11814

N/A - Documentation only

Checklist:

• I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• Link added in /components/index.rst when creating new documents for new components or cookbook.

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[Copilot]

Pull Request Overview

This PR adds comprehensive security best practices documentation for ESPHome users, providing practical guidance on securing devices and deployments. The document covers threat models, core security features, network security, physical security, secrets management, and incident response.

Key changes:

• New security best practices guide covering API encryption, OTA passwords, web server authentication, network segmentation, WiFi security, and more
• Cross-links added to relevant component documentation (API, OTA, web_server, WiFi, MQTT) to point users to the security guide
• FAQ entry added for quick access to security recommendations

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
content/guides/security_best_practices.md	New comprehensive security guide with threat model, best practices, configuration examples, and incident response procedures
content/guides/getting_started_hassio.md	Added cross-link to security guide in "See Also" section
content/guides/getting_started_command_line.md	Added cross-link to security guide in "See Also" section
content/guides/faq.md	Added new FAQ entry on securing ESPHome devices with link to security guide
content/components/wifi.md	Added TIP callout linking to WiFi security recommendations in the security guide
content/components/web_server.md	Added IMPORTANT callout emphasizing authentication requirement with link to security guide
content/components/ota/esphome.md	Added IMPORTANT callout about strong passwords with link to security guide
content/components/mqtt.md	Added TIP callout linking to MQTT security recommendations in the security guide
content/components/api.md	Added TIP callout linking to API encryption best practices in the security guide