F-070: design uniform target plugins API

[stepango]

Summary

• Accept design for uniform external Gradle plugins on Forma targets: single plugins: TargetPlugins argument on every target DSL.
• Deprecate chain UX (TargetBuilder.withPlugin / PluginWrapper) while keeping catalog dependency-driven plugins (KSP) and built-in features separate.
• Prefer single-arg config for complex cases; separate target types only when a plugin creates a new role.
• Board: F-070 done, F-071–F-073 todo (P7). Unblocks GH Documentation for external plugins configuration #36 docs story.

Design

See docs/TARGET-PLUGINS.md.

Target shape (end state)

androidRes(
    packageName = "…",
    dependencies = deps(androidx.navigation),
    plugins = plugins(
        plugin("androidx.navigation.safeargs.kotlin"),
    ),
)

androidBinary(
    /* … */,
    plugins = plugins(
        plugin("com.google.gms.google-services"),
        plugin("com.google.firebase.crashlytics", dependencies = google.firebase) {
            mappingFileUploadEnabled = false
        },
    ),
)

Test plan

• Docs-only — no product code
• Follow-up F-071 implements API + wires DSLs