Fix mermaid v10, bundle all mermaid code in component (#1190)

This PR does two things:

1. fixes using mermaid version `>= 10` from the CDN, by importing the ESM module instead
2. moves script loading code from `head.html` to the mermaid include

I've also added some light documentation to clarify how using mermaid with local paths should work (users should specify a version, and they should only use fully-minified bundles with no local references).

The nice thing about this approach is that it's a breaking change for nobody, and only adds functionality (v10 support). Eventually, we should remove support for mermaid <10, which should make this much easier!

Closes #1206.

## Context

In v10, Mermaid has implemented a few (admittedly, very frustrating to deal with) breaking changes:

1. they've removed CJS support, which is fine, *but*
2. that means that the `dist` they publish to JSDelivr now has a **different URL**: for versions `10.0.0` - `10.0.2`, **they do not have a minified bundle -- you have to load the ESM version with relative imports**
3. and, separately the `init` function has been deprecated

2 is really the issue, and so I've had to go into the code to now load mermaid by ESM by default when the user is on mermaid > v10.

I've tested this with:

- CDN version < 10 (v9)
- CDN version 10
- local path with version < 10 (v9)
- local path with version 10 (new: also loaded as an ESM module)

Separately, I chose to put all the mermaid stuff in one include because:

- I think @pdmosses requested something like this - it's a bit confusing that some mermaid code is *not* in the include, and this makes modular components ... more modular
- from a developer perspective, it's more clear what's happening with mermaid
- mermaid is not render-blocking, so it shouldn't be in the `head` anyways

---------

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

_includes/head.html

@@ -22,14 +22,6 @@
     <script src="{{ '/assets/js/vendor/lunr.min.js' | relative_url }}"></script>
   {% endif %}
 
-  {% if site.mermaid %}
-    {% if site.mermaid.path %}
-      <script src="{{ site.mermaid.path | relative_url }}"></script>
-    {% else %}
-      <script src="https://cdn.jsdelivr.net/npm/mermaid@{{ site.mermaid.version }}/dist/mermaid.min.js"></script>
-    {% endif %}
-  {% endif %}
-
   <script src="{{ '/assets/js/just-the-docs.js' | relative_url }}"></script>
 
   <meta name="viewport" content="width=device-width, initial-scale=1">