Docs: Add Callout to Stacks Helper page about limited flexbox gap support

[coliff]

Fixes: #34737

Preview screenshot from localhost

[XhmikosR]

If this only for one page, it doesn't need to be a partial

[coliff]

Do you think it's needed on the Gap utilities page too?
https://getbootstrap.com/docs/5.1/utilities/spacing/#gap
I personally don't think it's needed there as that section only talks about Grid gap (not flexbox gap)
cc @mdo

[XhmikosR]

I'll defer this to @mdo, but if it's only one place then we don't need the partial :)

[mdo]

Yeah we can skip the partial and do just the one page :)

[mdo]

Pre-approving for when the partial is removed :).

[mdo]

Thank you both!