docs(v3): drop redundant What's New list and remove stale @steipete credit (#221)

release-notes.md:
- Drop the "What's New" section entirely. It repeated the same items
  as the Headline features section above it in bulleted form, a
  holdover from the old v2.9 release notes pattern. CHANGELOG.md is
  the canonical Added/Changed/Fixed list; release notes is marketing
  copy and shouldn't duplicate it. Added a one-line pointer to
  CHANGELOG.md [3.0.0] for anyone looking for the detail.
- Rename "Credits" to "Earlier contributors" and note they are from
  the v1 and v2 lineage, so readers don't confuse them with v3
  contributors.
- Remove @steipete credit (did not actually contribute to this repo).

CHANGELOG.md [2.1.0] Credits:
- Remove @steipete credit (did not actually contribute to this repo).

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

CHANGELOG.md

@@ -189,7 +189,6 @@ Three headline features: watchlists for always-on bots, YouTube transcripts as a
 
 ### Credits
 
-- @steipete -- Bird CLI (vendored X search) and yt-dlp/summarize inspiration for YouTube transcripts
 - @galligan -- Marketplace plugin inspiration
 - @hutchins -- Pushed for YouTube feature