refactor: remove obsolete template marketplace

[zqr10159]

What changed

• remove the standalone template-marketplace/hertzbeat-template-hub service
• remove its separately maintained Angular frontend
• remove the current template marketplace help page and obsolete source archive rule
• keep the versioned 1.7/1.8 documentation and shared screenshots so historical documentation remains reproducible

Why

The standalone template marketplace is no longer needed in the current HertzBeat product direction. It is outside the main HertzBeat reactor and carries a separate backend, authentication model, storage configuration, dependency baseline, and frontend application.

Removing it avoids maintaining an unsupported parallel product while HertzBeat standardizes the main codebase on Java 25, Spring Boot 4, and the primary frontend. This change does not modify the main server modules.

Impact

• removes 244 files and approximately 26,000 lines of obsolete marketplace code and current documentation
• removes no module from the main Maven reactor
• preserves historical release documentation

Validation

• ./mvnw -q validate
• pnpm run md-lint — 0 errors
• Docusaurus production build for en and zh-cn
• current-code/current-documentation reference scan
• git diff --cached --check

AI assistance: used for draft implementation and test iteration.
Human validation: Java 25 Maven validation, Markdown lint, and two-locale documentation production builds completed successfully.
Risk notes: versioned 1.7/1.8 marketplace documentation and its screenshots are intentionally retained as historical artifacts.