docs(markdown-basics.qmd): add display math requirement

[mcanouil]

This pull request adds a note to the Markdown authoring documentation to clarify the rules for display math delimiters.

This is an important requirement that I think should be clearly stated as it is on Pandoc. https://pandoc.org/MANUAL.html#extension-tex_math_dollars

Documentation update:

• docs/authoring/markdown-basics.qmd: Added a callout note explaining that there should be no blank lines between the opening and closing $$ delimiters in display math.

[cscheid]

Thanks, this is a good idea. I wonder if we should have a special callout type in this page for syntax gotchas.

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