[docs] Add gap property documentation for Flexbox

[yousoumar]

• I have followed (at least) the PR section of the contributing guide.

There is no documentation for gap property in Flexbox page. gap is an important property that works for Grid and Flexbox in the same way. This PR adds the missing documentation.

[mui-bot]

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[oliviertassinari]

There is a mismatch between what we say in https://mui.com/material-ui/getting-started/supported-platforms/#browser and https://caniuse.com/flexbox-gap. The mismatch is with Safari. We support v12.5 while flex gap is only available since v14.5 Considering:

https://caniuse.com/flexbox-gap

and

https://gs.statcounter.com/ios-version-market-share/mobile/worldwide/#quarterly-201903-202204

It seems that it's indeed time that we start using it. I have added this breaking change idea to #30660.

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On a different note, since our last major was over a year ago. I think that it could even make sense to have a small v6 BCs cycle, something easy to upgrade to (not saying that we should do it, but it doesn't sound unreasonable)

[yousoumar]

Hi @oliviertassinari! Thanks for the insights! I didn't know about this support thing.