docs: include mermaid.js library

[lewismiddleton]

Adds the minified mermaid.js library to the mkdocs extra_javascript.

Inspired by a comment on the original feature request at mkdocs-material for mermaid support.

Using the extra_javascript feature to include the minified mermaid.js library from https://unpkg.com/browse/mermaid@10.9.0/dist/

Resolves: #567

[lewismiddleton]

Here are some findings from my testing:

• this did not work when using the fence_code_format see rabbit hole on div vs code
• adding extra_css did not change behaviour

markdown_extensions:
  - pymdownx.superfences:
      custom_fences:
        - name: mermaid
          class: mermaid
          format: !!python/name:pymdownx.superfences.fence_div_format
          # Doesn't work
          # format: !!python/name:pymdownx.superfences.fence_code_format

extra_javascript:
  - https://unpkg.com/mermaid@10.9.0/dist/mermaid.min.js

# Doesn't change behaviour
# extra_css:
#   - https://unpkg.com/mermaid@10.9.0/dist/mermaid.css