docs: update HOWTO‑add‑model.md for ModelBase and new model classes#14874

CISC merged 1 commit intoggml-org:masterggml-org/llama.cpp:masterfrom

wooksong:fix-typo-how-to-add-modelwooksong/llama.cpp:fix-typo-how-to-add-modelCopy head branch name to clipboard