docs(mtmd): add multimodal model development guide (ggml-org#17, docs-only)

Cherry-pick of the docs portion of c0a31b5 only: adds
docs/spacemit-mtmd-model-development.md (HF VLM -> GGUF text + ONNX vision split
and SMT pipeline development) and links it from docs/spacemit-mtmd.md.

The OHOS build-slimming and CI-disable hunks from the original commit are dropped
here — this K3 fork does not carry the OpenHarmony (OHOS) target (ggml-org#16 skipped).

(cherry picked from commit c0a31b5, docs hunks only)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: 2 people