[docs-infra] Fix arbitrary gap between paragraphs in callouts

[oliviertassinari]

Fix a regression introduced in #38525. I noticed the problem in #40909.

Before https://deploy-preview-40909--material-ui.netlify.app/material-ui/guides/nextjs/#installing-the-dependencies

After https://deploy-preview-40911--material-ui.netlify.app/experiments/docs/callouts/#multilines

[mui-bot]

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https://deploy-preview-40911--material-ui.netlify.app/

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[danilo-leal]

Looks good! I just don't understand why this is considered a regression. What was broken with the previous code? Maybe I'm not understanding, or it's worth clarifying in the PR description — not obvious from the screenshot diff. It looks more like an improvement than a bug fix to me.

[oliviertassinari]

What was broken with the previous code?

@danilo-leal The #38525 PR introduced a custom gap between paragraphs: https://github.com/mui/material-ui/pull/38525/files#diff-77f04bc58bbbb50bc7d3dea9ec533b08d76f110c581114cda8105da367d236a5R283

The CSS no longer relies on the default margin-bottom value. I think it's a regression because:

• It doesn't look consistent, or correct (too short of a gap, it should be at least larger than the line gap inside the same paragraph).
• It adds unnecessary complexity, defining the gap for all the typography seems enough.

This PR reverts to how it was before.