The public headers saw lots of cleanups, backporting from SDL3 docs, and
merging with the wiki.
The markdown files in docs/README-*.md were converted to Unix endlines.
* Xbox GDK support (14 squashed commits)
* Added basic keyboard testing
* Update readme
* Code review fixes
* Fixed issue where controller add/removal wasn't working (since the device notification events don't work on Xbox, have to use the joystick thread to poll XInput)