Make the docs logo readable on light backgrounds (#5389)

Currently, the logo on the docs site blends into the background on the
light theme:

<img width="320px"
src="https://github.com/user-attachments/assets/4cded445-d3af-4100-bd11-6a33da836cb1">


[Here](https://internals.rust-lang.org/t/feedback-on-new-rust-documentation-logo-background/12787)
is a discussion related to this. There doesn't seem to be a perfect way
to handle this situation.

---

Silent because the docs crate is not part of the regular release
process.

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Co-authored-by: ordian <[email protected]>