Add plantuml gem (#3005)

docs/Gemfile

@@ -2,6 +2,7 @@ source "https://rubygems.org"
 
 group :jekyll_plugins do
   gem "github-pages"
+  gem "jekyll-plantuml"
 end
 
 group :development do

docs/Gemfile.lock

@@ -130,6 +130,8 @@ GEM
     jekyll-optional-front-matter (0.3.2)
       jekyll (>= 3.0, < 5.0)
     jekyll-paginate (1.1.0)
+    jekyll-plantuml (1.4.2)
+      jekyll (> 2.0)
     jekyll-readme-index (0.3.0)
       jekyll (>= 3.0, < 5.0)
     jekyll-redirect-from (0.16.0)
@@ -261,6 +263,7 @@ PLATFORMS
 
 DEPENDENCIES
   github-pages
+  jekyll-plantuml
   webrick
 
 BUNDLED WITH

docs/_config.yml

@@ -10,3 +10,6 @@ aux_links:
     - https://core-design-history.herokuapp.com
   "GitHub":
     - https://github.com/communitiesuk/submit-social-housing-lettings-and-sales-data
+gems: ["jekyll-plantuml"]
+plugins:
+  - jekyll-diagrams