Docs: remove non-essential CHANGELOG information

[mattxwang]

Closes #1510.

Some things I'll note:

• I've added a callout for each minor release that has breaking and/or potentially breaking changes, but am light on the details (this seems to mirror what I've seen with other projects, e.g. ESLint/Stylelint/React, which link to a migration guide).
• I've elected to undo the short-cut markdown links in the CHANGELOG, mostly motivated by wanting to match the many other CHANGELOGs I read when writing this PR (presumably, they don't use this feature since it's not a part of CommonMark Markddown). Happy to undo this change if we'd like (it's a regex).
• I've otherwise removed basically all content that is not related to a PR (a "line item" in a CHANGELOG). This includes the preamble, upgrading/pinning comments, and new contributors.
  • (thinking about the future of some of these in #1510)
• I've made a subheading in MIGRATION for each minor release, and just noting when there are no breaking changes. I think this is helpful for readers (who may just think that the page is out of date).
• I've occasionally edited the messages to make them clearer, but only at the individual word level
• I've elected to not touch the releases from 0.3.3 (in part because I personally do not have sufficient context to edit/group/deduplicate them), but I could if desired.

Things left to do (outside of discussing in #1510):

• add a date to each release
• put the migration/pin information somewhere (README versus index versus both? the README also links to index for "usage")