docs: model files build locally — hub-referenced models not supported (#1495)

[LukasWodka]

Documents the #1495 contract on the Customize Models page (join-use-case/model-optimization.mdx):

• New "Models build locally (no external hubs)" section: a model file must build its architecture from local code; tracebloc does not fetch models/weights/tokenizers/configs from a hub (HuggingFace) at training time. No from_pretrained("<hub-id>"), no model_id/tokenizer_id/hf_token. Start from pretrained by downloading weights once and uploading them; ship a tokenizer.json for NLP.
• Removes the retired model_id / hf_tokenadditional variables (they now fail upload) and points to the new section.
• Adds the strict weight-load note (uploaded weights load into the architecture your file builds, matching names/shapes).

⚠️ DRAFT — do not merge until the block ships to prod. docs develop auto-deploys to production, and this describes behavior that isn't live until backend#1505 + tracebloc-py-package#380 reach prod. Un-draft + merge once they're deployed.

Refs #1151 · Documents #1495.

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[saqlainsyed007]

Content LGTM — accurate, and cleanly scoped. Verified:

• Both internal anchors resolve: #use-pre-trained-weights → the existing Use Pre-trained Weights heading, and the self-reference #models-build-locally-no-external-hubs → this new section's heading.
• The retired variables are fully removed — no dangling model_id / hf_token / tokenizer_id references remain anywhere in the doc; the only mentions left are the two that deliberately name them as no-longer-supported.
• The new section documents the #1495 contract at the right place (the canonical Customize Models page), and the strict weight-load Note matches the "build the same architecture" requirement.

Holding the formal approval (commenting, not approving) purely on your own prod-timing gate: docs develop auto-deploys to production, and this describes behavior that isn't live until backend#1505 + tracebloc-py-package#380 reach prod. The content is ready — approve/merge once those are deployed and you un-draft, per your note.