Add MermaidJS plugin

[antonpirker]

Problem Statement

Graphs and diagrams in the docs (be it application or develop docs) are right now created in another tool, and then the image is linked in our docs.

This leads to outdated graphs everywhere.

Solution Brainstorm

It would be great if we could create/change Mermaid graphs directly in the .mdx files.

So it is possible to add something like this directly in the docs (develop and other docs) and the graph is rendered on the page:

```mermaid
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
```

The default styles of the graphs should also be updated, because right now mermaid graphs are not readable that good in dark mode. See:

• https://develop.sentry.dev/application/architecture/
• https://develop.sentry.dev/backend/cross-region-replication/