fix(placement): adjust hover style for links

[caugner]

Summary

Problem

The "Mozilla ads" and "Don't want to see ads?" are not always recognizable as links, because they don't always change their text-decoration on hover.

Solution

Make sure that the text-decoration changes on hover.

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Screenshots

Before

Screen.Recording.2023-11-30.at.14.33.11.mov

After

Screen.Recording.2023-11-30.at.14.34.40.mov

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How did you test this change?

Ran yarn dev with REACT_APP_KUMA_HOST=developer.allizom.org and REACT_APP_PLACEMENT_ENABLED=true in my .env and looked at http://localhost:3000/en-US/docs/Web/HTML, then compared with main.

[LeoMcA]

code lgtm, I'll let you test when you add screenshots 🚀

[caugner]

Noticed two issues:

1. The ad text in the sidebar gets underlined on hover (might actually be desired, but this change was not intended).
2. The underline of the "Mozilla ads" text in the top banner does not toggle on hover.

edit: Fixed now.

[LeoMcA]

Code looks good, but a super opinionated view (so feel free to ignore) on the "Mozilla ads" underline (I think it looks bad):

[caugner]

Self-approval, and tested again:

1. In the top, both "Mozilla ads" and "Don't want to see ads?" are underlined only on hover.
2. In the sidebar, "Mozilla ads" is underlined except on hover, and "Don't want to see ads?" is underlined only on hover.
3. In the footer, "Mozilla ads" is underlined except on hover, and "Don't want to see ads?" is underlined only on hover.

Bottom line is this now works as expected, and code changes have already been reviewed by Leo.