Make Markdown flavour explicit on the markdown page

[mlevison]

📚 Subject area/topic

Markdown

📋 Page(s) affected (or suggested, for new content)

https://docs.astro.build/en/guides/markdown-content/

📋 Description of content that is out-of-date or incorrect

The page mentions the default markdown type in passing. It code state explicitly, the default markdown flavour is GitHub Flavoured and then link to the spec.

🖥️ Reproduction in StackBlitz (if reporting incorrect content or code samples)

No response