Ensure to pass 4.5:1 contrast ratio everywhere

Created by: ffoodd

Sounds like dependabot one, but isn't :)

Paving the way in #30468 — and to keep going with #29315 : my goal is to make this happen after tackling every obstacles.

To solve before the bump

Those should probably adressed separately, consider this as a roadmap to enlightment :D

• #30989 → .btn-success:hover case mentionned in #30468 (comment);
  • #30972 is maybe the wrong direction too—there mght be no need to change colors at all.
  • #31276 did it 🎉
• #30622 → color-level(), lighten() and darken() seem misused: either on :hover for buttons (always darken) or to define color and background-color with the same level, no matter what the base color is (eg. .alert-light seems wrong) — there's probably room to improvement by using tint() and shade() here;
  • Relates to #30487 (closed)
• #30550 → .text-info, .text-black-50 and .text-white-50 can't make it to 4.5:1 — however, using $gray-900 instead of $gray-800 for $dark would solve this (and several other dark-ish things with insufficient contrasts);
• A few components use light grays as background (mostly in cards and navbars) without accounting for it in their color usage, resulting in insufficient contrasts when used with .text-muted or .disabled things;
• Some .btn-outline-* and .link-* examples should be shown with dark background:
  1. #30044 will help to associate a contrast color for the docs part, through the data layer
  2. #30551 was a wrong direction but helped to define a visual consideration for examples with multiple backgrounds
  3. TODO will need another PR after #30044 to tackle this, I guess.

Merging this would be a kind of consecration 🎉

Fixes #25126 (closed)