docs: Add marker to signal which languages are built into Zed (#26913)

I saw over the weekend some social media posts that indicated people
didn't know which languages are included in Zed by default. We do say
that on each language-specific page, but I figured having this
high-level view on the languages page wouldn't hurt.

Release Notes:

- N/A
6cac0b33dcDanilo Leal committed on 3/17/2025, 2:27:52 PM· committed by GitHubparent45606ab
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