docs website & mkdocs.yml: redirect /branches/ URL

After this is merged, https://martinvonz.github.io/jj/prerelease/branches/
should redirect to https://martinvonz.github.io/jj/prerelease/bookmarks/
instead of resulting in a 404 error and old links to docs
(from posts or from Google index) breaking.

mkdocs-offline.yml

@@ -12,6 +12,7 @@ plugins:
     - search
     - redirects:
         redirect_maps:
+            branches.md: bookmarks.md
 # Turns out the `offline` plugin forces the following
 # option no matter what, and therefore shouldn't be
 # used in the main config file.

mkdocs.yml

@@ -48,6 +48,7 @@ plugins:
     - search
     - redirects:
         redirect_maps:
+            branches.md: bookmarks.md
 
 # Not all of these may be necessary, especially since the material
 # theme substitutes for some of them