Fixes versions content for 19, 20 and 21

_fc-versions/calcium.md

@@ -13,8 +13,10 @@ releases:
         help_link: /html/installations/calcium.html
       - name: User manual
         link: /download/user-manual-20.0-Calcium.pdf
+      - name: Plugin-In development guide
+        link: /download/plugin-development-guide-20.0-Calcium.pdf
         help: Hello plug-in tutorial archive
-        help_link: download/hello-20.0-Calcium.tar.gz
+        help_link: /download/hello-20.0-Calcium.tar.gz
       - name: API Documentation
         link: /download/frama-c-20.0-Calcium-api.tar.gz
       - name: ACSL 1.14 (Calcium implementation)

_fc-versions/potassium.md

@@ -13,8 +13,10 @@ releases:
         help_link: /html/installations/potassium.html
       - name: User manual
         link: /download/user-manual-19.1-Potassium.pdf
+      - name: Plugin-In development guide
+        link: /download/plugin-development-guide-19.1-Potassium.pdf
         help: Hello plug-in tutorial archive
-        help_link: download/hello-19.1-Potassium.tar.gz
+        help_link: /download/hello-19.1-Potassium.tar.gz
       - name: API Documentation
         link: /download/frama-c-19.1-Potassium-api.tar.gz
       - name: ACSL 1.14 (Potassium implementation)

_fc-versions/scandium.md

@@ -13,8 +13,10 @@ releases:
         help_link: /html/installations/scandium.html
       - name: User manual
         link: /download/user-manual-21.1-Scandium.pdf
+      - name: Plugin-In development guide
+        link: /download/plugin-development-guide-21.1-Calcium.pdf
         help: Hello plug-in tutorial archive
-        help_link: download/hello-21.1-Scandium.tar.gz
+        help_link: /download/hello-21.1-Scandium.tar.gz
       - name: API Documentation
         link: /download/frama-c-21.1-Scandium-api.tar.gz
       - name: ACSL 1.15 (Scandium implementation)