nx-site: add nx-api page to crates sidebar navigation

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Expose the existing nx-api crate reference page in the docs sidebar under Crates.

Why

• the page exists but is not easy to discover from navigation
• this creates an inconsistent experience in crate documentation
• the change is small and isolated to docs navigation config

Scope

• update docs/nx-site/astro.config.mjs
• add reference/crates/nx-api to the Crates sidebar group
• keep ordering/style coherent with existing entries

Done when

• nx-api is visible in the docs sidebar Crates section
• link resolves correctly to the existing page
• docs build/check passes

Notes

Please avoid unrelated sidebar reshuffling.

Thanks again for considering contributing to Numax.