Remove KeyboardTransform, convert `:kbd:` to a real role

[AA-Turner]

Is your feature request related to a problem? Please describe.

KeyboardTransform.run takes 1.2% of the runtime of a build. It was added in #7530 (#7593) to support sequences of keys, but shouldn't be needed if :kbd: is converted to a proper role.

Describe the solution you'd like

• Convert :kbd: to a SphinxRole
• Remove KeyboardTransform

Additional context

• Convert :kbd: to nested <kbd> HTML elements #7530
• Close #7530: html: Support nested <kbd> elements #7593
• :kbd: role with Caps Lock rendered incorrectly #8714
• Fix #8714: html: kbd role with "Caps Lock" rendered incorrectly #8719