docs: correct changeset guidance in CONTRIBUTING

[joshuaellis]

Why

The changeset workflow was documented only under "Publishing (Maintainers only)", which wrongly implied contributors don't add changesets and told everyone to "flag them all". Both contradict how the repo actually works: contributors add a changeset in their PR, and the fixed: [["@react-spring/*"]] group in .changeset/config.json version-locks every package, so flagging one bumps them all.

What

Document changeset creation as a contributor step under "Sending a PR" with the correct "flag only the package(s) you changed" guidance, and simplify the release flow to just bump and publish (the changesets already exist from merged PRs).

[changeset-bot]

⚠️ No Changeset found

Latest commit: ffd4cac

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR