Update package discovery guidance

[ekamran]

Summary

Updates handbook package docs that still pointed people toward the deprecated WP-CLI Package Index.

Changes

• Refer to Packagist for discovering standalone WP-CLI packages.
• Replace “Package Index” wording with Composer package wording.
• Remove the old Package Index repository example from the shared Composer project setup.
• Use wp-cli/server-command as the example package instead of runcommand/hook.

Verification

• Confirmed wp-cli/server-command is available on Packagist as a wp-cli-package.
• Confirmed the old Package Index guidance is only left in generated command docs, not the hand-written guide pages.
• Ran git diff --check.