[docs]feature: add H2 database (testing-only) guidance

[mohit07dec]

What's changed?

• Added a new documentation page explaining HertzBeat’s default embedded H2 usage and clearly stating it is for testing/sandbox only (not recommended for production).
• Documented a key security risk: if an attacker can execute SQL against H2, features like CREATE ALIAS may allow arbitrary Java code execution.
• Linked to official H2 security guidance and pointed users to the existing MySQL/PostgreSQL migration docs for production deployments.
• Updated docs sidebars so the H2 page appears under Quickstart → change-db (current docs + v1.6.x), including zh-cn translations.

Checklist

• I have read the Contributing Guide (docs changes only).
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed. (Not applicable: documentation-only change.)

Add or update API

• I have added the necessary e2e tests and all cases have passed. (Not applicable: docs-only change; no API behavior changes.)

[tomsun28]

hi please help fix the doc ci error