VSCode admonition styling using 'Markdown Extended'

[ewels]

Use https://github.com/qjebbs/vscode-markdown-extended to render admonitions in VSCode.

Requires this extension:

• Name: Markdown Extended
• Id: jebbs.markdown-extended
• Description: Extended syntaxes to built-in markdown & What you see is what you get exporter.
• Version: 1.1.4
• Publisher: jebbs
• VS Marketplace Link: https://marketplace.visualstudio.com/items?itemName=jebbs.markdown-extended

Then does an imperfect but not-too-bad job of rendering admonitions in markdown previews in VSCode:

Currently set to use workspace VSCode settings. This won't work when writing docs in pipeline repos. If we want to do that, we'll need to use a https URL to the stylesheet (not a problem once this is merged) and then add a .vscode directory to the template with this settings file (which I like less).

We could then include this VSCode extension into the nf-core VSCode package so that the majority of users will have it automagically.

'cc @FloWuenne @mahesh-panchal