Add conceptual documentation for SwiftPM plugins

[Viditgupta-official]

This PR adds a conceptual overview of Swift Package Manager plugins.

It explains:

• The two kinds of plugins supported by SwiftPM (command plugins and build tool plugins)
• When and how each type is invoked
• How plugin outputs are handled by the build system
• Visibility and limitations of generated outputs between plugins

This documentation clarifies common points of confusion around plugin execution and output visibility, and addresses the questions raised in #8469.