Set site_url so mermaid.js loads correctly

When site_url isn't set (or is falsey) the privacy plugin will include
mermaid.js on _every page_ to mitigate an issue with offline site
generation.

squidfunk/mkdocs-material#3742

mkdocs.yml

@@ -1,4 +1,5 @@
 site_name: Lick The Salt
+site_url: https://lickthesalt.com/
 docs_dir: content
 nav:
   - About: index.md
@@ -53,7 +54,11 @@ markdown_extensions:
   - pymdownx.keys
   - pymdownx.smartsymbols
   - pymdownx.snippets
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
   - pymdownx.tabbed:
       alternate_style: true
   - pymdownx.tasklist: