Fix #693: Add uniform bottom margin to h1 headings

[Chetan175]

Concisely describe the change

Fixes #693
This PR resolves an issue where # (h1) headings in Markdown files were lacking a bottom margin, causing the text directly underneath to stick too closely to the heading

The fix adds mb-6 to the h1 definition in the global @layer base of main.css. This ensures that all auto-generated markdown h1 tags get a uniform bottom margin, while safely allowing custom UI components (like HeroHeader and PageHeader) to override it with their own explicit utility classes.

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[antedotee]

I believe, it would be helpful to check if there are any other places where this fix is needed, to make sure this bug doesn't appear again. Just my opinion 😅