Add gap between tabset tabs for improved spacing

[clicktodev]

Introduce a gap between tabset tabs to enhance visual spacing and improve user experience.

[github-actions]

📦 Docs artifacts are ready: https://github.com/elixir-lang/ex_doc/actions/runs/12835388228/artifacts/2448793392

[josevalim]

I prefer them without spacing, so let's ask others' opinions! cc @DavidOliver

[clicktodev]

I prefer the one with spacing because it gives the elements room to breathe and is more aligned with the theme thanks to the rounded corners.
Also the original design was also broken despite the small border radius because you could see the rounded corners clashing with each other due to the lack of spacing

[josevalim]

Oh, I see there is a discussion on #2023. Given both @DavidOliver and I prefer the old one, I will revert and use z-index to ensure the highlighted one is on top. Thanks @hichemfantar !

[clicktodev]

I wish i could change your mind on the matter because from a design standpoint, snapping together rounded corners (even tiny ones) is a no go.
Essentially no spacing works when the corners are sharp because they snap to each other.
However with round corners, the principle is to go with spacing.
@DavidOliver @josevalim @jonatanklosko @liamcmitchell