Add Hugo Markdown-style alert support and docs

[chalin]

• Contributes to Add support for Hugo (GFM, Obsidian) Markdown alert syntax #2390
• Adds support for Hugo's Markdown-syntax of alerts, with a few Docsy extensions
• Adds preliminary docs for this kind of alert syntax, with examples. More content regarding how to customize alert styles will come later
• Reworks the "Page content" section of Adding Content, simplifying it and adding the alerts content

Preview: https://deploy-preview-2443--docsydocs.netlify.app/docs/content/adding-content/#alerts

Screenshot

[chalin]

/cc @vitorvasc @svnm - FYI, this is an improved version of what we have on the OTel site.