[DOCS] C snippets use nonexistent WasmEdge_ModuleInstance, WasmEdge_Result_Error and WasmEdge_ValType_I32 #320

Description

Some C snippets in the docs use API names that don't exist in the WasmEdge C API, so they fail to compile when copied:

1. The "create module instances from plug-ins" snippet declares WasmEdge_ModuleInstance *ModCxt. The type is WasmEdge_ModuleInstanceContext; WasmEdge_ModuleInstance has never existed. This snippet appears in docs/embed/use-case/plugin/c_sdk.md, docs/embed/c/reference/latest.md, the versioned references 0.12.x.md to 0.17.x.md, and upgrade_to_0.12.0.md (with the same copies under i18n/zh).
2. The "Securing the Plugin" snippets in docs/contribute/plugin/best_practice.md use:
   • WasmEdge_Result_Error. The only predefined results are WasmEdge_Result_Success, WasmEdge_Result_Terminate and WasmEdge_Result_Fail.
   • In[0].Type != WasmEdge_ValType_I32. Since 0.14.0, WasmEdge_ValType is a struct, and the value type is checked with WasmEdge_ValTypeIsI32().

I compiled the snippets with gcc against the 0.18.1 release headers:

error: unknown type name 'WasmEdge_ModuleInstance'; did you mean 'WasmEdge_ModuleInstanceContext'?
error: 'WasmEdge_ValType_I32' undeclared (first use in this function); did you mean 'WasmEdge_ValTypeIsI32'?
error: 'WasmEdge_Result_Error' undeclared (first use in this function); did you mean 'WasmEdge_Result_Fail'?

Screenshots

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Additional information

Fix: use WasmEdge_ModuleInstanceContext, WasmEdge_ValTypeIsI32(In[i].Type) and WasmEdge_Result_Fail in these snippets.