feat: add mermaid docs and sensible defaults

[calebdw]

Hello!

Changes:

• add mermaid theme docs
• change default mermaid bg to transparent (this seems to be how Github renders mermaid charts)
• add sensible mermaid default based on theme

I noticed that the mermaid theme parameters didn't make it into the documentation so I decided to add it.

I also tried to add sensible mermaid theme defaults to better match the supported themes instead of always having to override the theme. For instance, on the dark theme the original mermaid defaults (bg: white, theme: default) are quite jarring:

But updating to (bg: transparent, theme: dark) looks much nicer:

Thanks!

[calebdw]

@mfontanini, I used neutral on the lighter "dark" themes because they seemed to not cause as much contrast:

Here is neutral:

And here is dark

But I can update all of them

[mfontanini]

Hmm can you try a sequenceDiagram? The text in between the participants was showing up too light for me

[calebdw]

Sure!

Here's the neutral:

and here's the dark:

Thanks for mentioning, dark does seem to better choice here :+1:

[mfontanini]

Hmm now that I remember... When I was testing this I was trying it with pie charts. It's possible those look like crap in dark mode, I'm not sure. I'm away from computer now so I can't test those.

[calebdw]

So here's neutral:

and here's dark:

I think dark is the best sensible default here, note that it's always possible to customize individual charts if desired/necessary:

```mermaid +render
%%{init: {'theme':'forest'}}%%
  graph TD
    a --> b
```

[mfontanini]

So weird, I wonder what exact combo I tried. Maybe I missed dark and only tried neutral. Anyhow, this looks great. Thanks!

[calebdw]

Thank you sir!