Document bundler-specific UI plugin kit entry points

[ruibaby]

What this PR does

• Document the dedicated Vite and Rsbuild entry points introduced by Add dedicated Vite and Rsbuild bundler kit entry points halo#10232.
• Update plugin and theme examples to import configuration helpers from their build-system-specific entry points.
• Include the corresponding Vue build plugins in installation commands.
• Document the deprecation of the package-root exports and their planned removal in Halo 2.27.0.

Related change

• Add dedicated Vite and Rsbuild bundler kit entry points halo#10232

Validation

• corepack pnpm@10.33.0 lint
• corepack pnpm@10.33.0 build
• git diff --check