fix: skip to canvas link style

[yamanoku]

ref #15740

What I did

#15740 (comment)

SkipLink to canvas in the sidebar may show the button if the logo is vertical. This is the state of the screenshot you quoted.

There is a way to turn it off with display: none, but it makes it impossible for screen reader users to take advantage of it, so I modified the original style.

<style>
/* hide annoying "skip to canvas" button which is rendered over the logo (brand) image  */
.sidebar-header > a {
    display: none;
}
</style>

Screenshots

Changed to display with opacity.

2023-02-02.10.39.31.mov

How to test

Tests created with #15740 should work without problems.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[yamanoku]

This style is Visually Hidden. It is an element while still allowing it to be exposed to assistive technologies (such as screen readers).

ref: https://getbootstrap.com/docs/5.0/helpers/visually-hidden/