Use a gray background when hovering SelectMenu items

[simurai]

This is a follow-up for https://github.com/github/github/pull/130408 and moves the changes "downstream".

• Fix hover of selected tabs
• Use gray background when an item gets hovered/focused
• Split filter and footer in the docs (bonus)

Here the biggest visual change: Replace the blue background with gray:

Before	After

Motivation

The main motivation behind the change to use a gray background is to avoid clashing of colors. On dotcom there are cases where SelectMenu items can contain colored icons or color swatches. Using a light gray background makes sure those colors stand out enough.

Note: Dropdown menus are unchanged and keep the blue background.

Concerns

• GitHub's iconic "blue on hover" is lost.
• The hover effect with gray is also more subtle and maybe harder to see.

[vercel]

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[emilybrick]

This lgtm 🙌