Adding Mermaid support

I just added Mermaid support for posts in this blog!

Now,

sequenceDiagram
  actor User
  participant UI
  
  User ->> UI : press button
  UI -->> User : stop it

Can be rendered as:

This is all thanks to this guide I found: https://realfiction.net/posts/mermaid-in-astro/

I added this for my latest article: Understanding Systems with Sequence Diagrams.