Add LFM2.5-350M model card (DOC-24)

[tuliren]

Summary

Adds a model card for LFM2.5-350M, the smallest model in the LFM2.5 text family.

• New page lfm/models/lfm25-350m.mdx modeled on lfm25-1.2b-instruct.mdx, with HF / GGUF / MLX / ONNX badges and Transformers / llama.cpp / vLLM / SGLang quickstarts. Sampling params (temperature=0.1, top_k=50, repetition_penalty=1.05) match the published model card.
• New card on lfm/models/text-models.mdx under the LFM2.5 Models section ("Fastest" badge).
• New row in the model chart on lfm/models/complete-library.mdx.
• New row in the maintainer-only MODEL-MATRIX.md.

LFM2-350M is intentionally left in place (no deprecation badge) — same convention used when LFM2.5-VL-450M was added alongside LFM2-VL-450M.

Linear ticket: DOC-24

Review & Testing Checklist for Human

• Verify the model description, sampling parameters, and badge links match what marketing/research want to publish.
• Confirm whether LFM2-350M should be deprecated alongside this addition (parallel to how LFM2-1.2B was deprecated when LFM2.5-1.2B-Instruct was added). I left it as-is, mirroring the LFM2-VL-450M precedent.
• Run npx mintlify dev and click through /lfm/models/text-models, /lfm/models/lfm25-350m, and /lfm/models/complete-library to confirm rendering.

Notes

• Did not touch the docs.json banner (still announces LFM2.5-VL-450M); update separately if a launch announcement is desired.
• All four HF repos referenced in the new page (LFM2.5-350M, -GGUF, -MLX-8bit, -ONNX) were verified to exist (200 OK) before linking.

Link to Devin session: https://app.devin.ai/sessions/8d7fac6ebe234dd5a8a0d338de0475b4
Requested by: @tuliren

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