Reorganize the Explainer Explainer, take 2

[jyasskin]

This is based on @torgo's #39, with the following significant changes:

• Incorporate most of the comments from the PR review. In particular @LeaVerou's Revision to explainer explainer #39 (comment) and @alice's request to retain several of the original details.
• Avoid adding content in the same PR that reorganizes the existing content. (In particular, the comments on using Markdown. They'll come in a separate PR.)
• Avoid gratuitous changes to existing text, especially when the diff-ignoring-whitespace can match up the original text.
• Put the Key Components in the same order as the template, and take non-components out of that ordered list.
• Use more structure than just an ordered list in the Tips section.

I didn't take a couple comments from the original review, mostly because they're additions that we should discuss in a dedicated PR:

• Ask for multi-stakeholder review
• Show the user experience, end-to-end
• Developer-facing materials or research <- probably worth mentioning in the alternatives considered section and a new Tips section about how to show the Practical Use Cases are actually an improvement
• A "why write an explainer" section
• "Alternatives Considered", which should also change in the template.
• More detail on why to focus on the end-user's need

The template also needs some updates to match this. Since it's already out of date, let's align on this PR, and then I'll send another one to improve the template.