docs: Add a light border to h2s (#24554)

I was finding hard to navigate the "Configuring Zed" page with just
white space creating a boundary between the different chunks of content.
I think a slight border below the h2 heading helps a lot with that!

Release Notes:

- N/A
3f0288e52aDanilo Leal committed on 2/10/2025, 12:43:51 PM· committed by GitHubparentd15a61a
2 files changedLine totals unavailable