mermaid: refactor config to use mermaid_config.js include, only require mermaid.version in _config.yml (#909)

This PR has a bit of scope creep! This PR now:

- changes the mermaid opt-in logic to only check for the existence of a `mermaid` key instead of `mermaid != false`: this resolves the follow-up in #857
- changes the behaviour of mermaid configuration
    - instead of using `mermaid_init.html` with default settings, makes the include `mermaid_config.js`
    - the include is loaded directly into the contents of `mermaid_initialize`
    - by default, it is an empty object (i.e. `{}`), triggering the defaults
- updates docs
- adds an example to the markdown kitchen sink  

It does significantly change the interface provided in #857, and I apologize for the confusion. However, given the discussion in this PR, I think it's the best move forward!

_includes/head.html

@@ -30,7 +30,7 @@
     <script type="text/javascript" src="{{ '/assets/js/vendor/lunr.min.js' | relative_url }}"></script>
   {% endif %}
 
-  {% if site.mermaid_enabled != false %}
+  {% if site.mermaid %}
     <script src="https://cdn.jsdelivr.net/npm/mermaid@{{ site.mermaid.version }}/dist/mermaid.min.js"></script>
   {% endif %}