docs: Add multiple backend support to C API docs

docs/c-api/backends.md

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+# Multiple Backends in the C API
+
+Wasmer's C API now supports multiple backends, allowing you to choose the most appropriate WebAssembly runtime for your specific use case.
+
+## Available Backends
+
+The C API supports the following backends:
+
+- **UNIVERSAL** (sys): The default backend that supports multiple compilers (Cranelift, LLVM, Singlepass)
+- **V8**: Google's V8 JavaScript engine
+- **WAMR**: WebAssembly Micro Runtime
+- **WASMI**: A WebAssembly interpreter written in Rust
+- **JSC**: JavaScriptCore engine
+
+## Selecting a Backend
+
+You can select a backend when creating a new engine configuration:
+
+
+
+## Default Backend
+
+The default backend is determined by the features enabled during compilation. The priority order is:
+
+1. If a specific `*-default` feature is enabled (e.g., `sys-default`, `v8-default`, etc.), that backend is used
+2. Otherwise, the first available backend in the following order is used: sys, wamr, wasmi, v8, js, jsc
+
+## Engine-Specific Configuration
+
+Each backend has its own specific configuration options. The C API now provides engine-specific functions to configure each backend.
+
+### Universal (sys) Backend Configuration
+
+The Universal backend supports multiple compilers and configuration options:
+
+
+
+## Breaking Changes
+
+This update introduces several breaking changes to the C API:
+
+1. Functions that were implicitly tied to the `sys` engine are now explicitly scoped with the `sys` prefix:
+   - `wasm_config_canonicalize_nans` → `wasm_config_sys_canonicalize_nans`
+   - `wasm_config_set_target` → `wasm_config_set_sys_target`
+   - `wasm_config_push_middleware` → `wasm_config_sys_push_middleware`
+   - `wasm_config_set_compiler` → `wasm_config_set_sys_compiler`
+
+2. Calling engine-specific configuration functions will automatically switch the engine type to the corresponding backend. For example, calling `wasm_config_sys_push_middleware` will set the engine to `UNIVERSAL` regardless of what was previously set.
+
+## Feature Flags
+
+To use a specific backend, you need to enable the corresponding feature when building Wasmer:
+
+- `sys`: Enable the Universal backend
+- `v8`: Enable the V8 backend
+- `wamr`: Enable the WAMR backend
+- `wasmi`: Enable the WASMI backend
+- `jsc`: Enable the JSC backend
+
+To set a specific backend as default, use the corresponding `-default` feature:
+
+- `sys-default`: Use the Universal backend as default
+- `v8-default`: Use the V8 backend as default
+- `wamr-default`: Use the WAMR backend as default
+- `wasmi-default`: Use the WASMI backend as default
+- `jsc-default`: Use the JSC backend as default
+
+## Example
+