Mermaid support

[rphair]

@fill-the-fill this parallels cardano-foundation/cf-cip-frontend#26 for a similarly derived site (cips.cardano.org) (cc @KtorZ)

We're starting to get more CIPs using Mermaid and eventually authors will come to expect it, particularly for flowcharts / state diagrams and often for sequence diagrams. As these other changes are in the works:

• Proposal to Rebrand Cardano Developer Portal to how.cardano.org #1292 cc @katomm
• [WIP] Upgrade to Docusaurus 3 #1157 cc @Mercurial

... can we add integration with the Mermaid library to the list of essential improvements? Currently only one merged CIP uses Mermaid (without rendering, dumping as an unexpected code block):

• https://developers.cardano.org/docs/governance/cardano-improvement-proposals/CIP-0045
  • (rendered) https://github.com/cardano-foundation/CIPs/blob/master/CIP-0045/README.md

... but we have more on the way currently under review:

• CIP-0113? | Programmable tokens CIPs#444 (added Mermaid recently)
• CIP-0137? | Decentralized Message Queue CIPs#876 (submitted with it)
• ... some other PRs I think: will list here as we find more