Improve "references" coverage.

1. Introduce TXT, MD, and DOC section partials for "references".
2. Reference TXT/MD partials from Basis theme so inherited themes
(Modern, Compact, Positive) see the changes.

themes/basis/src/md.txt

@@ -23,3 +23,5 @@
 {{> section/recognition }}
 
 {{> section/speaking }}
+
+{{> section/references }}

themes/basis/src/txt.txt

@@ -21,3 +21,5 @@
 {{> section/service }}
 
 {{> section/recognition }}
+
+{{> section/references }}