docs: refresh the README

[ogorzalka]

Restructures the README on the shared Pollora layout.

• Banner header with version, downloads, Tests and license badges; intro paragraph.
• "Part of Pollora" line: in a Pollora project, post types and taxonomies are declared with #[PostType] / #[Taxonomy] and registered through this package.
• Emoji feature bullets replaced by plain ones; note that make() registers on init.
• Documentation section links the local docs and the pollora.dev Post types / Taxonomies pages.
• Testing section fixed: the helpers live in tests/Helpers/, and composer test also runs PHPStan and Pint.
• Contributing and License footer; LICENSE holder line now links https://rubee.group.