docs: make landing pages more informative

[dwilding]

The purpose of this PR is to use a consistent style for the three landing pages How-to guides, Explanation, and Reference. How-to guides already has the style we want, so I'm using the same style for Explanation and Reference.

Summary of changes:

• How-to guides - Adjusted some wording for readability and consistency of language
• Explanation - Moved General model to the top of the TOC; split the landing page into sections with brief summaries
• Reference - Applied a logical order to the TOC; split the landing page into sections with brief summaries

[benhoyt]

Thanks, this is very good -- just requesting a few nit comments/changes.

In addition, I think we should discuss the ToC order. I think the "natural" or "logical" order is good in the main document, but (especially as things get longer) alphabetical order would be better for scanning/finding things in the ToC on the left. But let's discuss at/after daily.

[dwilding]

Thanks @benhoyt! I've included your suggestions in dwilding@e90e04e

Let's discuss the TOC approach today

[benhoyt]

Great, thanks. Feel free to merge as is and figure out the ToC ordering in a separate PR, or include that in this one -- doesn't matter to me either way.

[dwilding]

@benhoyt, @IronCore864 I've reorganized the links after our discussion. Changes in 261e6da:

• Links under Reference are now listed alphabetically in the nav sidebar
• On the Reference page, I've bumped a few sections to the top, for a more logical reading order. And I feel that grouping auto-restart & health checks together makes for a better explanation
• Minor adjustments to a couple of link titles:
  • Use Pebble in containers --> Pebble in containers
  • CLI Commands --> CLI commands
• Added comments in the source to explain how Reference is set up

Let me know if you think anything should be changed!