Docs: standardize usage of `<kbd>`

Description

This PR proposes the standardize the usage of <kbd> along the documentation:

• <kbd><kbd>Ctrl</kbd> + <kbd>Key</kbd></kbd> when combination of keys
• <kbd>Key</kbd> for simple keys
• Don't use full uppercase keys ("TAB" -> "Tab", etc.)

Motivation & Context

Consistency throughout the documentation and use standard name of the keys.

Type of changes

• Enhancement (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly

Live previews

• Dropdowns > Accessibility // Dropdowns > Options
• Popover > Usage
• Content > Reboot > User input
• Migration > Documentation changes

• Check js/tests/visual/button.html manually

Related issues

N/A