Distinguish between Wasm runtime and execution environment

[kate-goldenring]

The ecosystem could benefit from establishing terminology to explain the difference between a Wasm runtime and a project that uses a Wasm runtime under the hood. For example, both Spin and WasmCloud execute Wasm using the Wasmtime runtime. What do we call those projects? "Execution environment" is one option.

[vados-cosmonic]

Another option here is "Embedding" -- it's a bit of an abstract term (I might prefer "execution environment") but both spin and wasmCloud are wasm runtime embeddings/ wasmtime embeddings.

Maybe it makes sense to add a section to 6 (6.3)? and cover "custom embeddings" or "custom execution environments" ? We could go over how to do it generically and point to the host code that is actually already in the project that we use to run the "adder".