docs: add MariaDB in readme

[chr-hertel]

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[Copilot]

Pull Request Overview

This PR adds MariaDB support to the project documentation by including a code example and listing MariaDB in the Supported Stores section.

• Add a MariaDB similarity-search code example link
• Insert MariaDB in the Supported Stores list and adjust ChromaDB ordering

Comments suppressed due to low confidence (2)

README.md:460

• [nitpick] List items under '#### Code Examples' use two-space indentation before the numbering; add leading spaces to align with existing entries.

1. [MariaDB Store](examples/store/mariadb-similarity-search.php)

README.md:468

• [nitpick] Supported Stores list items start with a space before the asterisk for consistent Markdown formatting; add a space before *.

* [MariaDB](https://mariadb.org/projects/mariadb-vector/) (requires `ext-pdo`)