Improve flex list item padding (#26779)

Improve flex list item padding (#26779)

Replace #26761

It's better to keep children elements simple, and let parent containers
layout the necessary padding/margin.

The old `not(:last-child)` and `.flex-item + .flex-item` are not easy to
maintain (for example, what if the developer would like to use a "tiny
height" item?)

The old approach also makes some UI look strange because the first item
doesn't have proper padding-top.

In this PR, we just simply use `.flex-item { padding: ... }`:

* Developers could manually set the item height they want easily
* It's easier to make it work with various containers -- with padding
(`ui segment`) and without padding (`div`)

And added more samples/examples.

![image](https://github.com/go-gitea/gitea/assets/2114189/719ea712-0241-4426-b67f-5723993c4ed7)

Co-authored-by: Giteabot <teabot@gitea.io>