Docs: consistent usage of CSS sections

[julien-deramond]

Description

Based on 20ab821 and our recent modifications around custom properties, some sections are sometimes called Sass, Sass variables, CSS, etc.

Based on similar files in these folders, this PR suggests modifications to have consistent "CSS" + "Sass variables" sections in:

• /components
• /content
• /forms

Based on similar files in these folders, this PR suggests modifications to have consistent "Sass mixins" sections in:

• /components

Based on similar files in these folders, this PR suggests modifications to have consistent "Sass loop" sections in:

• /components

Warning
Not knowing exactly the global approach, this PR doesn't modify remaining ## Sass$ occurrences in /helpers, /layout, and /utilities.

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38175--twbs-bootstrap.netlify.app/docs/5.3/components/dropdowns/#sass-mixins
• https://deploy-preview-38175--twbs-bootstrap.netlify.app/docs/5.3/components/modal/#sass-loop
• https://deploy-preview-38175--twbs-bootstrap.netlify.app/docs/5.3/components/placeholders/#css
• https://deploy-preview-38175--twbs-bootstrap.netlify.app/docs/5.3/content/images/#css
• https://deploy-preview-38175--twbs-bootstrap.netlify.app/docs/5.3/forms/floating-labels/#css

[louismaximepiton]

I think some are missing in:

• ratio.md, not sure about this one
• containers.md
• css-grid.md
• almost each in utilities/**.md

We should also unify our ## Utilities API title ?

[julien-deramond]

Yep, this is what I've mentioned in the description, in the warning message. This PR treats only the things I'm sure about but I can modify the rest of it of course if we state exactly what direction we want to take. @mdo thoughts here?

[mdo]

@julien-deramond If you want to update those other pages, including the API heading, feel free!

[julien-deramond]

So I'm merging this one already reviewed and I'll create another PR so that I can suggest something.
Edit: See #38219