Promote _Backend to public api #19605

Problem

Discussed on gitter here: https://gitter.im/matplotlib/matplotlib?at=603c83fc44f5a454a447c6e9

The _Backend class added in #8773 would be useful for external backends such as ipympl, and is already used by mplcairo. But it is currently marked as a private.

Proposed Solution

1. Make _Backend public
2. Document _Backend as stable.