docs: consolidate design change logs and refresh stale docs

features/sites_backend.feature

@@ -6,7 +6,7 @@ Feature: Sites backend, the owner manages sites and every article belongs to exa
   So that a single back-office can hold the content of more than one website
 
   Background:
-    Given the spec of record is docs/design/screens/sites.md, docs/design/states.md (site rows), docs/design/routes.md and docs/design/sites-authors-slices.md
+    Given the spec of record is docs/design/screens/sites.md, docs/design/states.md (site rows), docs/design/routes.md and docs/design/decisions.md
     And this slice is backend only: no SPA, no change to docs/design
     And a Site has id (uint32), name, key, base_url, default_social_image_id (nullable), default_robots ("index" or "noindex"), locale (nullable), is_default and article_count, in snake_case JSON
     And exactly one site is the default at any time, enforced by the use case inside a transaction and not by a partial index