Document cNF, phosphoproteomics, and phosphosites on the docs site

[jjacobson95]

The new cNF dataset, the phosphoproteomics data type, and the phosphosites reference need documentation:

• Dataset page/entry for cNF (sources, modalities, sample types).
• Schema docs for the Phosphosite / Phosphoproteomics classes and phosphosite_id.
• API-reference entries for the new loadable attributes (phosphoproteomics, phosphosites).

Complements existing docs issues (#337 datasets page, #216 figures, #475 API reference rendering).