Add higher-level reasons to the "Why the Component Model" section

[rylev]

The "Why the Component Model" section answers the question of why the component model exists from first principles (i.e., what direct problems of only having wasm modules does the component model solve?). This is a great resource, but it misses a some of the higher level features that the component model provides. For example, the component model offers the ability to compose components together without those components themselves having any knowledge of this composition.

Before the current explanation of why the component model exists, we should enumerate the feature set that the component model has that WebAssembly modules do not have.

[kate-goldenring]

+1. This is a great point. Maybe even a table/diagram of the intersection benefits of core wasm and components and the benefits you only get from CM. @rylev could we add this to why-component-model.md, or do you imagine it having its own doc?

[rylev]

I imagined this fitting nicely in why-component-model.md. If at some point that page becomes too long, we can break it out.