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Docs: touch responsiveness optimization

Admittedly wordy info dump on why we use `touch-action:manipulation`

refs https://github.com/twbs/bootstrap/pull/18051#issuecomment-156375970

Add touch-action:manipulation on interactive elements to avoid 300ms click delay

Block level cards

fixes #18241: bump up border-radius values on .label-pill for when customization means the 1rem value doesn't cut it

Document links with text emphasis classes

Autoprefixer callout in Build tools docs

Checkbox validation markup (Take 2)

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Refactoring according the comments on https://github.com/twbs/bootstrap/pull/18104.
Closes #18131

Add shrink-to-fit to responsive meta section

Closes #18068

Fixes #18044

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