Update Markdown parser to be on-par with Azure Wiki Markdown

[ricardoboss]

According to this page in the Microsoft docs, the syntax for embedding snippets (like videos or mermaid diagrams) and adding a TOC is a bit different than we currently use: https://learn.microsoft.com/en-us/azure/devops/project/wiki/wiki-markdown-guidance?view=azure-devops-2019

This raises two issues:

• Update TOC generator to only insert the TOC at the [[__TOC__]] mark #55
• Add a snippets scanner (:::<snippet>) and convert mermaid diagrams to only work inside of them #56

[ricardoboss]

This would also prepare features for use by #9

[ricardoboss]

Closing this in favour of the more general #63