Revising the Octane Upgrade Guide

[jenweber]

If you're someone with push access to my PR, please feel free to edit directly whenever possible by adding commits rather than comments. It's much appreciated!

Here's what I changed, broadly speaking:

• Remove references to components as "glimmer components"
• Remove most of the sections explaining editions, since it's covered in our new main site pages
• Added some more structure. Each major section has "benefits" and "getting used to..."
• Edited for conciseness where possible
• Fixed up some small accessibility things
• reformatted very long bulleted lists
• took out typescript reference

If this seems good, I can follow up with a second pass that aims to replace about half the code examples with links to existing sections in the Guides. I think many of the sections of the Components explanations can be a quick TLDR and advising to read the section of the "real" Guides in full.

[MelSumner]

A lot of edits...phew! Great job! 👍

[rwjblue]

@jenweber - Any objections to landing? Happy to keep iterating but this has lots of good fixes already...

[locks]

Round 2, begin!