Use example package names in plugin documentation

[ruibaby]

Summary

• Replace Halo-owned package paths and Gradle group IDs in plugin examples with com.example namespaces.
• Update sample inter-plugin dependency coordinates to avoid suggesting that plugin developers use the run.halo namespace.
• Apply the changes to the current documentation and versioned documentation from Halo 2.20 through 2.25.

Testing

• pnpm lint
• pnpm build
• git diff --check