docs: consolidate design change logs and refresh stale docs

features/media_field_library.feature

@@ -7,7 +7,7 @@ Feature: Media field usage in the Media library
     Given the SPA calls the API with relative URLs under /v1 and the "jwt" HttpOnly cookie
     And API errors are plain text
     And this slice changes only web/ (and web/visual); the backend of features/media_field_type.feature is done and not touched
-    And the design is docs/design/media-field-changes.md section 3.8 and section 4, with the board group "11 Media library: usage" (4 states) of "RenewCMS Screens Schema.dc.html", which draws MediaFrame.dc.html with entries counted as uses
+    And the design is docs/design/media-field.md section 3.8 and section 4, with the board group "11 Media library: usage" (4 states) of "RenewCMS Screens Schema.dc.html", which draws MediaFrame.dc.html with entries counted as uses
     And GET /v1/media/ items carry "used_in", which counts articles, authors, sites and entries together, and no split
     And GET /v1/media/{id}/usage answers an array ordered articles, authors, sites, then entries by content type name then entry id, with no total and no "first few" wrapper
     And an article item is {"type":"article","id","title","published","fields"}