docs: add tinydbjoin and tinydbstorage to extensions

[FerdinaKusumah]

Adds two extensions to the list in docs/extensions.rst, both mine and both on PyPI:

• tinydbjoin (repo, PyPI): inner and left joins between TinyDB tables, and nesting related documents into their parent. It takes tables, so the two sides can sit in different databases or storages. It sticks to what "Why Not Use TinyDB?" leaves out on purpose by staying outside the core, and it never joins on doc_id unless asked, since a deleted document's doc_id is handed out again once the database is reopened.
• tinydbstorage (repo, PyPI): Redis and S3 storages (MinIO and Cloudflare R2 included). A write that would overwrite another process's change raises an error instead.

Both are marked beta: tinydbjoin is at 0.1.0 and tinydbstorage at 0.2.0. I put them after tinydb-rust, the other beta entry; happy to move them if you'd rather order the list differently.

The docs build with Sphinx 8.1 gives the same 5 warnings as on master, none from these entries.

[msiemens]

I've merged the PR, thanks for contributing to the TinyDB ecosystem!