Issues with curve-based animation migration guide

The migration guide (here: https://bevyengine.org/learn/migration-guides/0-14-to-0-15/#curve-based-animation):
1. includes a section from the PR written for the PR reviewers, which is not really part of the migration guide
2. more importantly, hasn't been updated with cart's rewrite of animation properties (bevyengine/bevy#16484)

I can probably get around to fixing it before too long myself, but I figured I'd post an issue in case anyone wants to pick it up.

Start with the curve-based animation section of the 0.14-to-0.15 migration guide at the linked URL, then read the referenced rewrite in bevyengine/bevy#16484. Remove the material intended only for PR reviewers and update the guide to reflect the rewritten animation properties. Done means the guide accurately matches the rewrite and remains useful to migration readers.

Written by the indexing model from the issue text.