extensions: Add sphinxcontrib-mermaid

To write mermaid diagrams, see: https://github.com/mgaitan/sphinxcontrib-mermaid.

For example:

```{mermaid}

   sequenceDiagram
      participant Alice
      participant Bob
      Alice->John: Hello John, how are you?
      loop Healthcheck
          John->John: Fight against hypochondria
      end
      Note right of John: Rational thoughts <br/>prevail...
      John-->Alice: Great!
      John->Bob: How about you?
      Bob-->John: Jolly good!

```

Author: tdgroot

docs/conf.py

@@ -44,6 +44,7 @@
     "sphinx_copybutton",
     "notfound.extension",
     "hypernode.sphinx.extensions.updated_at",
+    "sphinxcontrib.mermaid",
 ]
 
 # Add any paths that contain templates here, relative to this directory.

requirements/base.txt

@@ -10,6 +10,7 @@ mdformat-frontmatter==0.4.1
 sphinx-notfound-page==0.8.3
 sphinx-sitemap==2.4.0
 GitPython==3.1.30
+sphinxcontrib-mermaid==0.7.1
 
 # hypernode/ requirements
 beautifulsoup4==4.11.1