Improve introductory documentation

[littledan]

The API surface is motivated by use cases, and it should be possible to tell a strong user story for each of those use cases in the docs. This could include one usage per class and one usage per method name/category, as the recent presentation does. This could be presented in two ways:

• In the main page of the documentation, the basics could be explained in a way that's parallel to the presentation
• The cookbook could include examples of of using each main part of the API

[littledan]

Note that the TAG asked about an explainer in w3ctag/design-reviews#311 (comment) . I'm not really sure whether anything is needed, but a short document of some kind (could be largely copy-pasted from a blog post/with reused wording from the docs) explaining the motivation for the proposal and how Temporal meets it, especially being driven by code samples from the beginning, could be useful.