Update heading.js

[nsolerieu]

Increase margin and padding of headings for a improved reading experience

See issue here: https://github.com/primer/doctocat/issues/699

Changes:

• Margin top all heading to 1margin-top: 64px1 (previously 24px)
• Margin bottom of all heading increase by one step 16 -> 24
• Padding bottom of h1 and h2 adjusted to match margin bottom increase by one step 16 -> 24px

---

Previews:

H1 - H2

H3 - H4

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Latest commit: 1668601d6ca5696400ceddffed16d193f4489cce

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