docs: clarify custom theme build flow

[Thanhdn1984]

Summary

• add the missing post-alias step for custom template/theme builds
• show importing Converse from an app entrypoint and rebuilding the wrapper app
• document what a complete example repo should include

Fixes #2919

Verification

• git diff --check
• cd docs && npm ci && npm run build (fails before page build because root PostCSS config requires missing autoprefixer from repo-level deps)