add mfa doc (#26654) (#26674)

add mfa doc (#26654) (#26674)

Backport #26654 by @lunny

copy and modified from #14572

> Whilst debating enforcing MFA within our team, I realised there isn't
a lot of context to the side effects of enabling it. Most of us use Git
over HTTP and would need to add a token.

I plan to add another PR that adds a sentence to the UI about needing to
generate a token when enabling MFA if HTTP is to be used.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: silverwind <me@silverwind.io>