report: fix link contrast issue in dark mode

[SphinxKnight]

I went with --color-informative which was already in use for some other links and it looks (locally) ok to me in order to fix #10363.

Summary
This PR styles .lh-table-column--link a elements with the custom color --color-informative instead of the default (user agent stylesheet) color for links (see #10363 for rationale: contrast too low / a11y)

This is intended as a bugfix for #10363.

The need for this change is a better visual contrast in dark mode and respecting WCAG2.1 rules along the way.

https://www.w3.org/TR/WCAG21/#contrast-minimum

Related Issues/PRs
See #10363 for original issue.

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[SphinxKnight]

@googlebot I signed it!

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[patrickhulce]

Oh wow you already fixed it! I'll update my comment then :D great work thanks!

[SphinxKnight]

@connorjclark thanks again for the review (1 line, 25 chars approx. and I manage to slip two typos in... :s)

[connorjclark]

lgtm! thanks for sticking to it