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: Recommend that Explainers use Markdown or similar. #61.)
• 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: Ask for multi-stakeholder review in the Explainer Explainer #67
• Show the user experience, end-to-end: Ask for the end-to-end user experience in explainers. #66
• 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: Suggest linking to developer-facing materials or research #68
• A "why write an explainer" section: Explain why to write an explainer #69
• "Alternatives Considered", which should also change in the template: Replace "considered alternatives" with "alternatives considered". #64.
• More detail on why to focus on the end-user's need: Explain why to focus on the end-user's need, and/or moderate that advice #70

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 Since it's already out of date, let's align on this PR, and then I'll send another one to improve the template.

[torgo]

it looks good to me - I suggest we merge this and them address the additional issues you've pointed out in a separate PR

[alice]

I think this is a great re-write, thanks for working on it!

[alice]

Is there some reason to put this first?

[jyasskin]

It's been first in the template, and because it tends to be really short, that probably makes sense. Then it seemed weird to have this list be in a different order than the template, even though that takes it out of being ordered by importance. But I'm happy to reorder if you think this item makes more sense elsewhere in the list.

[hober]

Let's merge this! I'm looking forward to future improvements that build on this.