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.

[heckj]

@swift-ci please test

[heckj]

Hey Vidit,

Thanks for providing this - did you by chance see the content in Plugins.md, one directory above this?

I think rather than dropping in another article with the same name, which DocC will have some trouble with (it views everything in the catalog as a single namespace, and sort of ignores any folder/directory structure within it), you'd be better revising what's in the existing content to help answer the questions in #8469