Move plugin-architecture Piped usage guide to official documentation

*What would you like to be added:

Move the plugin-architecture Piped usage guide from:

cmd/pipedv1/README-usage-alpha.md

to the official documentation under:

docs/content/en/docs-v1.0.x/installation/install-piped/plugin-architecture.md

This would include:

• Migrating all relevant sections (Prerequisites, Setup Control Plane, Run Piped, Deploy an Application)
• Replacing the "alpha" notice with a proper Hugo alert block
• Updating internal links to align with documentation pages
• Converting the existing README into a pointer to the new documentation

Why is this needed:

• Improves discoverability, as users typically look for installation guidance in the official docs/ directory rather than inside cmd/
• Provides a more structured and complete documentation experience
• Reduces maintenance overhead by leveraging Hugo-based docs (versioning, search, consistent styling)
• Improves onboarding for new users and contributors by centralizing important usage instructions

Start by reading cmd/pipedv1/README-usage-alpha.md and the surrounding pages under docs/content/en/docs-v1.0.x/installation/install-piped/. Move the Prerequisites, Setup Control Plane, Run Piped, and Deploy an Application sections into plugin-architecture.md, update internal links and the alpha notice, then convert the README into a pointer to the new page. Done means the content is discoverable in the official documentation with consistent Hugo formatting and working links.