Clarify a bit web integration document

[nicolo-ribaudo]

I was reading through the web integration doc again, and changed some parts to highlight more the core design aspects and the reason behind them. Importantly, I added the same observations that there are in the slides for the upcoming TC39 meeting:

• the proposed behavior matches what you would usually get from userland implementations
• in many cases there is no difference between dispatch and causal contexts.

I also changed the note about Observers, marking that as Important using GitHub's syntax (https://github.com/orgs/community/discussions/16925), explaining why going through the always-empty-context path can be explained as coherent.

[nicolo-ribaudo]

I pushed a new commit removing most of the mentions to the registration context. It was defined and referred to all over the document, but almost never actually used in a way that is meaningfully different from the dispatch context. We also don't really need to define the dispatch context anymore, since we don't define its opposite.